diff --git a/dsl/src/ast.rs b/dsl/src/ast.rs
new file mode 100644
index 0000000..be188f0
--- /dev/null
+++ b/dsl/src/ast.rs
@@ -0,0 +1,31 @@
+use crate::*;
+
+#[derive(Debug, Clone, Default, PartialEq)]
+pub struct Ast(pub AstValue);
+
+/// The abstract syntax tree (AST) can be produced from the CST
+/// by cloning source slices into owned [Arc] values.
+pub type AstValue = DslValue<Arc<str>, VecDeque<Ast>>;
+
+//#[derive(Debug, Clone, Default, PartialEq)]
+//pub struct AstIter();
+
+impl<'src> From<Cst<'src>> for Ast {
+    fn from (token: Cst<'src>) -> Self {
+        token.value().into()
+    }
+}
+
+impl<'src> From<CstValue<'src>> for Ast {
+    fn from (value: CstValue<'src>) -> Self {
+        Self(match value {
+            DslValue::Nil       => DslValue::Nil,
+            DslValue::Err(e)    => DslValue::Err(e),
+            DslValue::Num(u)    => DslValue::Num(u),
+            DslValue::Sym(s)    => DslValue::Sym(s.into()),
+            DslValue::Key(s)    => DslValue::Key(s.into()),
+            DslValue::Str(s)    => DslValue::Str(s.into()),
+            DslValue::Exp(d, x) => DslValue::Exp(d, x.map(|x|x.into()).collect()),
+        })
+    }
+}
diff --git a/dsl/src/cst.rs b/dsl/src/cst.rs
new file mode 100644
index 0000000..3dff71f
--- /dev/null
+++ b/dsl/src/cst.rs
@@ -0,0 +1,290 @@
+//! The concrete syntax tree (CST) implements zero-copy
+//! parsing of the DSL from a string reference. CST items
+//! preserve info about their location in the source.
+
+use crate::*;
+
+/// Implement the const iterator pattern.
+macro_rules! const_iter {
+    ($(<$l:lifetime>)?|$self:ident: $Struct:ty| => $Item:ty => $expr:expr) => {
+        impl$(<$l>)? Iterator for $Struct {
+            type Item = $Item;
+            fn next (&mut $self) -> Option<$Item> { $expr }
+        }
+        impl$(<$l>)? ConstIntoIter for $Struct {
+            type Kind = IsIteratorKind;
+            type Item = $Item;
+            type IntoIter = Self;
+        }
+    }
+}
+
+/// Static iteration helper used by [cst].
+macro_rules! iterate {
+    ($expr:expr => $arg: pat => $body:expr) => {
+        let mut iter = $expr;
+        while let Some(($arg, next)) = iter.next() {
+            $body;
+            iter = next;
+        }
+    }
+}
+
+/// CST stores strings as source references and expressions as [SourceIter] instances.
+pub type CstValue<'source> = DslValue<&'source str, SourceIter<'source>>;
+
+/// Token sharing memory with source reference.
+#[derive(Debug, Copy, Clone, Default, PartialEq)]
+pub struct Cst<'src> {
+    /// Reference to source text.
+    pub source: &'src str,
+    /// Index of 1st character of token.
+    pub start:  usize,
+    /// Length of token.
+    pub length: usize,
+    /// Meaning of token.
+    pub value:  CstValue<'src>,
+}
+
+impl<'src> Cst<'src> {
+    pub const fn new (
+        source: &'src str,
+        start:  usize,
+        length: usize,
+        value:  CstValue<'src>
+    ) -> Self {
+        Self { source, start, length, value }
+    }
+    pub const fn end (&self) -> usize {
+        self.start.saturating_add(self.length)
+    }
+    pub const fn slice (&'src self) -> &'src str {
+        self.slice_source(self.source)
+    }
+    pub const fn slice_source <'range> (&'src self, source: &'range str) -> &'range str {
+        str_range(source, self.start, self.end())
+    }
+    pub const fn slice_source_exp <'range> (&'src self, source: &'range str) -> &'range str {
+        str_range(source, self.start.saturating_add(1), self.end())
+    }
+    pub const fn with_value (self, value: CstValue<'src>) -> Self {
+        Self { value, ..self }
+    }
+    pub const fn value (&self) -> CstValue<'src> {
+        self.value
+    }
+    pub const fn error (self, error: DslError) -> Self {
+        Self { value: DslValue::Err(error), ..self }
+    }
+    pub const fn grow (self) -> Self {
+        Self { length: self.length.saturating_add(1), ..self }
+    }
+    pub const fn grow_num (self, m: usize, c: char) -> Self {
+        match to_digit(c) {
+            Result::Ok(n)  => Self { value: DslValue::Num(10*m+n), ..self.grow() },
+            Result::Err(e) => Self { value: DslValue::Err(e),      ..self.grow() },
+        }
+    }
+    pub const fn grow_key (self) -> Self {
+        let token = self.grow();
+        token.with_value(DslValue::Key(token.slice_source(self.source)))
+    }
+    pub const fn grow_sym (self) -> Self {
+        let token = self.grow();
+        token.with_value(DslValue::Sym(token.slice_source(self.source)))
+    }
+    pub const fn grow_str (self) -> Self {
+        let token = self.grow();
+        token.with_value(DslValue::Str(token.slice_source(self.source)))
+    }
+    pub const fn grow_exp (self) -> Self {
+        let token = self.grow();
+        if let DslValue::Exp(depth, _) = token.value() {
+            token.with_value(DslValue::Exp(depth, SourceIter::new(token.slice_source_exp(self.source))))
+        } else {
+            unreachable!()
+        }
+    }
+    pub const fn grow_in (self) -> Self {
+        let token = self.grow_exp();
+        if let DslValue::Exp(depth, source) = token.value() {
+            token.with_value(DslValue::Exp(depth.saturating_add(1), source))
+        } else {
+            unreachable!()
+        }
+    }
+    pub const fn grow_out (self) -> Self {
+        let token = self.grow_exp();
+        if let DslValue::Exp(depth, source) = token.value() {
+            if depth > 0 {
+                token.with_value(DslValue::Exp(depth - 1, source))
+            } else {
+                return self.error(Unexpected(')'))
+            }
+        } else {
+            unreachable!()
+        }
+    }
+}
+
+pub const fn to_number (digits: &str) -> DslResult<usize> {
+    let mut value = 0;
+    iterate!(char_indices(digits) => (_, c) => match to_digit(c) {
+        Ok(digit) => value = 10 * value + digit,
+        Result::Err(e) => return Result::Err(e)
+    });
+    Ok(value)
+}
+
+pub const fn to_digit (c: char) -> DslResult<usize> {
+    Ok(match c {
+        '0' => 0, '1' => 1, '2' => 2, '3' => 3, '4' => 4,
+        '5' => 5, '6' => 6, '7' => 7, '8' => 8, '9' => 9,
+        _ => return Result::Err(Unexpected(c))
+    })
+}
+
+pub const fn peek_src <'src> (source: &'src str) -> Option<Cst<'src>> {
+    use DslValue::*;
+    let mut token: Cst<'src> = Cst::new(source, 0, 0, Nil);
+    iterate!(char_indices(source) => (start, c) => token = match token.value() {
+        Err(_) => return Some(token),
+        Nil => match c {
+            ' '|'\n'|'\r'|'\t' =>
+                token.grow(),
+            '(' =>
+                Cst::new(source, start, 1, Exp(1, SourceIter::new(str_range(source, start, start + 1)))),
+            '"' =>
+                Cst::new(source, start, 1, Str(str_range(source, start, start + 1))),
+            ':'|'@' =>
+                Cst::new(source, start, 1, Sym(str_range(source, start, start + 1))),
+            '/'|'a'..='z' =>
+                Cst::new(source, start, 1, Key(str_range(source, start, start + 1))),
+            '0'..='9' =>
+                Cst::new(source, start, 1, match to_digit(c) {
+                    Ok(c) => DslValue::Num(c),
+                    Result::Err(e) => DslValue::Err(e)
+                }),
+            _ => token.error(Unexpected(c))
+        },
+        Str(_) => match c {
+            '"' => return Some(token),
+            _ => token.grow_str(),
+        },
+        Num(n) => match c {
+            '0'..='9' => token.grow_num(n, c),
+            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
+            _ => token.error(Unexpected(c))
+        },
+        Sym(_) => match c {
+            'a'..='z'|'A'..='Z'|'0'..='9'|'-' => token.grow_sym(),
+            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
+            _ => token.error(Unexpected(c))
+        },
+        Key(_) => match c {
+            'a'..='z'|'0'..='9'|'-'|'/' => token.grow_key(),
+            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
+            _ => token.error(Unexpected(c))
+        },
+        Exp(depth, _) => match depth {
+            0 => return Some(token.grow_exp()),
+            _ => match c {
+                ')' => token.grow_out(),
+                '(' => token.grow_in(),
+                 _  => token.grow_exp(),
+            }
+        },
+    });
+    match token.value() {
+        Nil => None,
+        _ => Some(token),
+    }
+}
+
+/// Owns a reference to the source text.
+/// [SourceConstIter::next] emits subsequent pairs of:
+/// * a [Cst] and
+/// * the source text remaining
+///   * [ ] TODO: maybe [SourceConstIter::next] should wrap the remaining source in `Self` ?
+#[derive(Copy, Clone, Debug, Default, PartialEq)]
+pub struct SourceConstIter<'src>(pub &'src str);
+
+impl<'src> From<SourceConstIter<'src>> for SourceIter<'src> {
+    fn from (source: SourceConstIter<'src>) -> Self{
+        Self(source)
+    }
+}
+
+impl<'src> From<&'src str> for SourceConstIter<'src> {
+    fn from (source: &'src str) -> Self{
+        Self::new(source)
+    }
+}
+
+impl<'src> SourceConstIter<'src> {
+    pub const fn new (source: &'src str) -> Self {
+        Self(source)
+    }
+    pub const fn chomp (&self, index: usize) -> Self {
+        Self(split_at(self.0, index).1)
+    }
+    pub const fn next (mut self) -> Option<(Cst<'src>, Self)> {
+        Self::next_mut(&mut self)
+    }
+    pub const fn peek (&self) -> Option<Cst<'src>> {
+        peek_src(self.0)
+    }
+    pub const fn next_mut (&mut self) -> Option<(Cst<'src>, Self)> {
+        match self.peek() {
+            Some(token) => Some((token, self.chomp(token.end()))),
+            None => None
+        }
+    }
+}
+
+const_iter!(<'src>|self: SourceConstIter<'src>| => Cst<'src> => self.next_mut().map(|(result, _)|result));
+
+/// Provides a native [Iterator] API over [SourceConstIter],
+/// emitting [Cst] items.
+///
+/// [Cst::next] returns just the [Cst] and mutates `self`,
+/// instead of returning an updated version of the struct as [SourceConstIter::next] does.
+#[derive(Copy, Clone, Debug, Default, PartialEq)]
+pub struct SourceIter<'src>(pub SourceConstIter<'src>);
+
+impl<'src> SourceIter<'src> {
+    pub const fn new (source: &'src str) -> Self {
+        Self(SourceConstIter::new(source))
+    }
+    pub const fn peek (&self) -> Option<Cst<'src>> {
+        self.0.peek()
+    }
+}
+
+impl<'src> Iterator for SourceIter<'src> {
+    type Item = Cst<'src>;
+    fn next (&mut self) -> Option<Cst<'src>> {
+        self.0.next().map(|(item, rest)|{
+            self.0 = rest;
+            item
+        })
+    }
+}
+
+impl<'src> From<&'src str> for SourceIter<'src> {
+    fn from (source: &'src str) -> Self{
+        Self(SourceConstIter(source))
+    }
+}
+
+impl<'src> Into<Vec<Cst<'src>>> for SourceIter<'src> {
+    fn into (self) -> Vec<Cst<'src>> {
+        self.collect()
+    }
+}
+
+impl<'src> Into<Vec<Ast>> for SourceIter<'src> {
+    fn into (self) -> Vec<Ast> {
+        self.map(Into::into).collect()
+    }
+}
diff --git a/dsl/src/dsl.rs b/dsl/src/dsl.rs
new file mode 100644
index 0000000..efdb2fe
--- /dev/null
+++ b/dsl/src/dsl.rs
@@ -0,0 +1,100 @@
+use crate::*;
+
+#[derive(Error, Debug, Copy, Clone, PartialEq)] pub enum DslError {
+    #[error("parse failed: not implemented")]
+    Unimplemented,
+    #[error("parse failed: empty")]
+    Empty,
+    #[error("parse failed: incomplete")]
+    Incomplete,
+    #[error("parse failed: unexpected character '{0}'")]
+    Unexpected(char),
+    #[error("parse failed: error #{0}")]
+    Code(u8),
+}
+
+/// Thing that may construct itself from `State` and [DslValue].
+pub trait FromDsl<State>: Sized {
+    fn try_provide (
+        state: &State,
+        value: DslValue<impl DslStr, impl DslExp>
+    ) -> Perhaps<Self>;
+    fn provide (
+        state: &State,
+        value: DslValue<impl DslStr, impl DslExp>,
+        error: impl Fn()->Box<dyn std::error::Error>
+    ) -> Usually<Self> {
+        match Self::try_provide(state, value)? {
+            Some(value) => Ok(value),
+            _ => Err(error())
+        }
+    }
+}
+
+pub type DslResult<T> = Result<T, DslError>;
+
+/// Marker trait for supported string types.
+pub trait DslStr: PartialEq + Clone + Default + Debug + AsRef<str> {}
+impl<T: PartialEq + Clone + Default + Debug + AsRef<str>> DslStr for T {}
+
+/// Marker trait for supported expression types.
+pub trait DslExp: PartialEq + Clone + Default + Debug {}
+impl<T: PartialEq + Clone + Default + Debug> DslExp for T {}
+
+/// A DSL value generic over string and expression types.
+/// See [CstValue] and [AstValue].
+#[derive(Clone, Debug, PartialEq, Default)]
+pub enum DslValue<Str: DslStr, Exp: DslExp> {
+    #[default]
+    Nil,
+    Err(DslError),
+    Num(usize),
+    Sym(Str),
+    Key(Str),
+    Str(Str),
+    Exp(usize, Exp),
+}
+
+impl<Str: DslStr, Exp: DslExp> DslValue<Str, Exp> {
+    pub fn is_nil (&self) -> bool {
+        matches!(self, DslValue::Nil)
+    }
+    pub fn as_err (&self) -> Option<&DslError> {
+        if let DslValue::Err(e) = self { Some(e) } else { None }
+    }
+    pub fn as_num (&self) -> Option<usize> {
+        if let DslValue::Num(n) = self { Some(*n) } else { None }
+    }
+    pub fn as_sym (&self) -> Option<&str> {
+        if let DslValue::Sym(s) = self { Some(s.as_ref()) } else { None }
+    }
+    pub fn as_key (&self) -> Option<&str> {
+        if let DslValue::Key(k) = self { Some(k.as_ref()) } else { None }
+    }
+    pub fn as_str (&self) -> Option<&str> {
+        if let DslValue::Str(s) = self { Some(s.as_ref()) } else { None }
+    }
+    pub fn as_exp (&self) -> Option<&Exp> {
+        if let DslValue::Exp(_, x) = self { Some(x) } else { None }
+    }
+    pub fn exp_depth (&self) -> Option<usize> {
+        todo!()
+    }
+    pub fn exp_head (&self) -> Option<&Self> {
+        todo!()
+    } // TODO
+    pub fn exp_tail (&self) -> Option<&[Self]> {
+        todo!()
+    } // TODO
+    pub fn peek (&self) -> Option<Self> {
+        todo!()
+    }
+    pub fn next (&mut self) -> Option<Self> {
+        todo!()
+    }
+    pub fn rest (self) -> Vec<Self> {
+        todo!()
+    }
+}
+
+impl<Str: DslStr + Copy, Exp: DslExp + Copy> Copy for DslValue<Str, Exp> {}
diff --git a/dsl/src/dsl_ast.rs b/dsl/src/dsl_ast.rs
deleted file mode 100644
index c28d73a..0000000
--- a/dsl/src/dsl_ast.rs
+++ /dev/null
@@ -1,45 +0,0 @@
-use crate::*;
-use std::sync::Arc;
-
-#[derive(Debug, Clone, Default, PartialEq)]
-pub struct Ast(pub Value<Arc<str>, AstIter>);
-
-impl<'source> From<CstToken<'source>> for Ast {
-    fn from (token: CstToken<'source>) -> Self {
-        token.value().into()
-    }
-}
-
-impl<'source> From<CstValue<'source>> for Ast {
-    fn from (value: CstValue<'source>) -> Self {
-        use Value::*;
-        Self(match value {
-            Nil    => Nil,
-            Err(e) => Err(e),
-            Num(u) => Num(u),
-            Sym(s) => Sym(s.into()),
-            Key(s) => Key(s.into()),
-            Str(s) => Str(s.into()),
-            Exp(d, x) => Exp(d, x.into()),
-        })
-    }
-}
-
-impl DslValue for Ast {
-    type Str = Arc<str>;
-    type Exp = AstIter;
-    fn value (&self) -> &Value<Arc<str>, AstIter> {
-        self.0.value()
-    }
-}
-
-impl DslToken for Ast {
-    type Value = Self;
-    type Meta  = ();
-    fn value (&self) -> &Self::Value {
-        self
-    }
-    fn meta (&self) -> &Self::Meta {
-        &()
-    }
-}
diff --git a/dsl/src/dsl_cst.rs b/dsl/src/dsl_cst.rs
deleted file mode 100644
index cfae7e2..0000000
--- a/dsl/src/dsl_cst.rs
+++ /dev/null
@@ -1,183 +0,0 @@
-use crate::*;
-
-/// CST stores strings as source references and expressions as new [SourceIter] instances.
-pub type CstValue<'source> = Value<&'source str, SourceIter<'source>>;
-
-/// Token sharing memory with source reference.
-#[derive(Debug, Copy, Clone, Default, PartialEq)]
-pub struct CstToken<'source>(pub CstValue<'source>, pub CstMeta<'source>);
-
-/// Reference to the source slice.
-#[derive(Debug, Copy, Clone, Default, PartialEq)] pub struct CstMeta<'source> {
-    pub source: &'source str,
-    pub start:  usize,
-    pub length: usize,
-}
-
-impl<'source> CstToken<'source> {
-    pub const fn new (
-        source: &'source str, start: usize, length: usize, value: CstValue<'source>
-    ) -> Self {
-        Self(value, CstMeta { source, start, length })
-    }
-    pub const fn end (&self) -> usize {
-        self.1.start.saturating_add(self.1.length)
-    }
-    pub const fn slice (&'source self) -> &'source str {
-        self.slice_source(self.1.source)
-    }
-    pub const fn slice_source <'range> (&'source self, source: &'range str) -> &'range str {
-        str_range(source, self.1.start, self.end())
-    }
-    pub const fn slice_source_exp <'range> (&'source self, source: &'range str) -> &'range str {
-        str_range(source, self.1.start.saturating_add(1), self.end())
-    }
-    pub const fn with_value (self, value: CstValue<'source>) -> Self {
-        Self(value, self.1)
-    }
-    pub const fn value (&self) -> CstValue<'source> {
-        self.0
-    }
-    pub const fn error (self, error: DslError) -> Self {
-        Self(Value::Err(error), self.1)
-    }
-    pub const fn grow (self) -> Self {
-        Self(self.0, CstMeta { length: self.1.length.saturating_add(1), ..self.1 })
-    }
-    pub const fn grow_num (self, m: usize, c: char) -> Self {
-        use Value::*;
-        match to_digit(c) {
-            Result::Ok(n)  => Self(Num(10*m+n), self.grow().1),
-            Result::Err(e) => Self(Err(e), self.grow().1),
-        }
-    }
-    pub const fn grow_key (self) -> Self {
-        use Value::*;
-        let token = self.grow();
-        token.with_value(Key(token.slice_source(self.1.source)))
-    }
-    pub const fn grow_sym (self) -> Self {
-        use Value::*;
-        let token = self.grow();
-        token.with_value(Sym(token.slice_source(self.1.source)))
-    }
-    pub const fn grow_str (self) -> Self {
-        use Value::*;
-        let token = self.grow();
-        token.with_value(Str(token.slice_source(self.1.source)))
-    }
-    pub const fn grow_exp (self) -> Self {
-        use Value::*;
-        let token = self.grow();
-        if let Exp(depth, _) = token.value() {
-            token.with_value(Exp(depth, SourceIter::new(token.slice_source_exp(self.1.source))))
-        } else {
-            unreachable!()
-        }
-    }
-    pub const fn grow_in (self) -> Self {
-        let token = self.grow_exp();
-        if let Value::Exp(depth, source) = token.value() {
-            token.with_value(Value::Exp(depth.saturating_add(1), source))
-        } else {
-            unreachable!()
-        }
-    }
-    pub const fn grow_out (self) -> Self {
-        let token = self.grow_exp();
-        if let Value::Exp(depth, source) = token.value() {
-            if depth > 0 {
-                token.with_value(Value::Exp(depth - 1, source))
-            } else {
-                return self.error(Unexpected(')'))
-            }
-        } else {
-            unreachable!()
-        }
-    }
-}
-
-pub const fn to_number (digits: &str) -> DslResult<usize> {
-    let mut value = 0;
-    iterate!(char_indices(digits) => (_, c) => match to_digit(c) {
-        Ok(digit) => value = 10 * value + digit,
-        Result::Err(e) => return Result::Err(e)
-    });
-    Ok(value)
-}
-
-pub const fn to_digit (c: char) -> DslResult<usize> {
-    Ok(match c {
-        '0' => 0, '1' => 1, '2' => 2, '3' => 3, '4' => 4,
-        '5' => 5, '6' => 6, '7' => 7, '8' => 8, '9' => 9,
-        _ => return Result::Err(Unexpected(c))
-    })
-}
-
-/// Static iteration helper.
-#[macro_export] macro_rules! iterate {
-    ($expr:expr => $arg: pat => $body:expr) => {
-        let mut iter = $expr;
-        while let Some(($arg, next)) = iter.next() {
-            $body;
-            iter = next;
-        }
-    }
-}
-
-pub const fn peek_src <'a> (source: &'a str) -> Option<CstToken<'a>> {
-    use Value::*;
-    let mut token: CstToken<'a> = CstToken::new(source, 0, 0, Nil);
-    iterate!(char_indices(source) => (start, c) => token = match token.value() {
-        Err(_) => return Some(token),
-        Nil => match c {
-            ' '|'\n'|'\r'|'\t' =>
-                token.grow(),
-            '(' =>
-                CstToken::new(source, start, 1, Exp(1, SourceIter::new(str_range(source, start, start + 1)))),
-            '"' =>
-                CstToken::new(source, start, 1, Str(str_range(source, start, start + 1))),
-            ':'|'@' =>
-                CstToken::new(source, start, 1, Sym(str_range(source, start, start + 1))),
-            '/'|'a'..='z' =>
-                CstToken::new(source, start, 1, Key(str_range(source, start, start + 1))),
-            '0'..='9' =>
-                CstToken::new(source, start, 1, match to_digit(c) {
-                    Ok(c) => Value::Num(c),
-                    Result::Err(e) => Value::Err(e)
-                }),
-            _ => token.error(Unexpected(c))
-        },
-        Str(_) => match c {
-            '"' => return Some(token),
-            _ => token.grow_str(),
-        },
-        Num(n) => match c {
-            '0'..='9' => token.grow_num(n, c),
-            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
-            _ => token.error(Unexpected(c))
-        },
-        Sym(_) => match c {
-            'a'..='z'|'A'..='Z'|'0'..='9'|'-' => token.grow_sym(),
-            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
-            _ => token.error(Unexpected(c))
-        },
-        Key(_) => match c {
-            'a'..='z'|'0'..='9'|'-'|'/' => token.grow_key(),
-            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
-            _ => token.error(Unexpected(c))
-        },
-        Exp(depth, _) => match depth {
-            0 => return Some(token.grow_exp()),
-            _ => match c {
-                ')' => token.grow_out(),
-                '(' => token.grow_in(),
-                 _  => token.grow_exp(),
-            }
-        },
-    });
-    match token.value() {
-        Nil => None,
-        _ => Some(token),
-    }
-}
diff --git a/dsl/src/dsl_display.rs b/dsl/src/dsl_display.rs
deleted file mode 100644
index ab190b7..0000000
--- a/dsl/src/dsl_display.rs
+++ /dev/null
@@ -1,29 +0,0 @@
-use crate::*;
-
-impl Display for Ast {
-    fn fmt (&self, out: &mut Formatter) -> Result<(), FormatError> {
-        match &self.0 {
-            Value::Nil       => Ok(()),
-            Value::Err(e)    => write!(out, "[error: {e}]"),
-            Value::Num(n)    => write!(out, "{n}"),
-            Value::Sym(s)    => write!(out, "{s}"),
-            Value::Key(s)    => write!(out, "{s}"),
-            Value::Str(s)    => write!(out, "{s}"),
-            Value::Exp(_, e) => write!(out, "{e:?}"),
-        }
-    }
-}
-
-impl<'source> Display for CstValue<'source> {
-    fn fmt (&self, out: &mut Formatter) -> Result<(), FormatError> {
-        match self {
-            Value::Nil       => Ok(()),
-            Value::Err(e)    => write!(out, "[error: {e}]"),
-            Value::Num(n)    => write!(out, "{n}"),
-            Value::Sym(s)    => write!(out, "{s}"),
-            Value::Key(s)    => write!(out, "{s}"),
-            Value::Str(s)    => write!(out, "{s}"),
-            Value::Exp(_, e) => write!(out, "{e:?}"),
-        }
-    }
-}
diff --git a/dsl/src/dsl_error.rs b/dsl/src/dsl_error.rs
deleted file mode 100644
index ffb53fd..0000000
--- a/dsl/src/dsl_error.rs
+++ /dev/null
@@ -1,16 +0,0 @@
-use crate::*;
-
-pub type DslResult<T> = Result<T, DslError>;
-
-#[derive(Error, Debug, Copy, Clone, PartialEq)] pub enum DslError {
-    #[error("parse failed: not implemented")]
-    Unimplemented,
-    #[error("parse failed: empty")]
-    Empty,
-    #[error("parse failed: incomplete")]
-    Incomplete,
-    #[error("parse failed: unexpected character '{0}'")]
-    Unexpected(char),
-    #[error("parse failed: error #{0}")]
-    Code(u8),
-}
diff --git a/dsl/src/dsl_iter.rs b/dsl/src/dsl_iter.rs
deleted file mode 100644
index 2e0e47a..0000000
--- a/dsl/src/dsl_iter.rs
+++ /dev/null
@@ -1,130 +0,0 @@
-use crate::*;
-
-pub trait DslIter {
-    type Token: DslToken;
-    fn peek (&self) -> Option<<Self::Token as DslToken>::Value>;
-    fn next (&mut self) -> Option<<Self::Token as DslToken>::Value>;
-    fn rest (self) -> Vec<Self::Token>;
-}
-
-/// Implement the const iterator pattern.
-#[macro_export] macro_rules! const_iter {
-    ($(<$l:lifetime>)?|$self:ident: $Struct:ty| => $Item:ty => $expr:expr) => {
-        impl$(<$l>)? Iterator for $Struct {
-            type Item = $Item;
-            fn next (&mut $self) -> Option<$Item> { $expr }
-        }
-        impl$(<$l>)? ConstIntoIter for $Struct {
-            type Kind = IsIteratorKind;
-            type Item = $Item;
-            type IntoIter = Self;
-        }
-    }
-}
-
-/// Owns a reference to the source text.
-/// [SourceConstIter::next] emits subsequent pairs of:
-///   * a [CstToken] and
-///   * the source text remaining
-///     * [ ] TODO: maybe [SourceConstIter::next] should wrap the remaining source in `Self` ?
-#[derive(Copy, Clone, Debug, Default, PartialEq)]
-pub struct SourceConstIter<'source>(pub &'source str);
-
-impl<'source> From<SourceConstIter<'source>> for SourceIter<'source> {
-    fn from (source: SourceConstIter<'source>) -> Self{
-        Self(source)
-    }
-}
-
-/// Provides a native [Iterator] API over [SourceConstIter],
-/// emitting [CstToken] items.
-///
-/// [Cst::next] returns just the [CstToken] and mutates `self`,
-/// instead of returning an updated version of the struct as [SourceConstIter::next] does.
-#[derive(Copy, Clone, Debug, Default, PartialEq)]
-pub struct SourceIter<'source>(pub SourceConstIter<'source>);
-
-impl<'source> SourceIter<'source> {
-    pub const fn new (source: &'source str) -> Self {
-        Self(SourceConstIter::new(source))
-    }
-    pub const fn peek (&self) -> Option<CstToken<'source>> {
-        self.0.peek()
-    }
-}
-
-impl<'source> Iterator for SourceIter<'source> {
-    type Item = CstToken<'source>;
-    fn next (&mut self) -> Option<CstToken<'source>> {
-        self.0.next().map(|(item, rest)|{self.0 = rest; item})
-    }
-}
-
-impl<'source> From<&'source str> for SourceIter<'source> {
-    fn from (source: &'source str) -> Self{
-        Self(SourceConstIter(source))
-    }
-}
-
-impl<'source> Into<Vec<CstToken<'source>>> for SourceIter<'source> {
-    fn into (self) -> Vec<CstToken<'source>> {
-        self.collect()
-    }
-}
-
-impl<'source> Into<Vec<Ast>> for SourceIter<'source> {
-    fn into (self) -> Vec<Ast> {
-        self.map(Into::into).collect()
-    }
-}
-
-impl<'source> From<&'source str> for SourceConstIter<'source> {
-    fn from (source: &'source str) -> Self{
-        Self::new(source)
-    }
-}
-
-impl<'source> SourceConstIter<'source> {
-    pub const fn new (source: &'source str) -> Self {
-        Self(source)
-    }
-    pub const fn chomp (&self, index: usize) -> Self {
-        Self(split_at(self.0, index).1)
-    }
-    pub const fn next (mut self) -> Option<(CstToken<'source>, Self)> {
-        Self::next_mut(&mut self)
-    }
-    pub const fn peek (&self) -> Option<CstToken<'source>> {
-        peek_src(self.0)
-    }
-    pub const fn next_mut (&mut self) -> Option<(CstToken<'source>, Self)> {
-        match self.peek() {
-            Some(token) => Some((token, self.chomp(token.end()))),
-            None => None
-        }
-    }
-}
-
-const_iter!(<'source>|self: SourceConstIter<'source>| => CstToken<'source> => self.next_mut().map(|(result, _)|result));
-
-#[derive(Debug, Clone, Default, PartialEq)]
-pub struct AstIter(std::collections::VecDeque<Ast>);
-
-impl DslIter for AstIter {
-    type Token = Ast;
-    fn peek (&self) -> Option<Ast> {
-        self.0.get(0).cloned()
-    }
-    fn next (&mut self) -> Option<Ast> {
-        self.0.pop_front()
-    }
-    fn rest (self) -> Vec<Ast> {
-        self.0.into()
-    }
-}
-
-impl<'source> From<SourceIter<'source>> for AstIter {
-    fn from (source: SourceIter<'source>) -> Self {
-        Self(source.map(Into::into).collect())
-    }
-}
diff --git a/dsl/src/dsl_test.rs b/dsl/src/dsl_test.rs
deleted file mode 100644
index 2f5ce15..0000000
--- a/dsl/src/dsl_test.rs
+++ /dev/null
@@ -1,111 +0,0 @@
-use crate::*;
-
-#[cfg(test)] mod test_token_iter {
-    use crate::*;
-    //use proptest::prelude::*;
-    #[test] fn test_iters () {
-        let mut iter = crate::SourceIter::new(&":foo :bar");
-        let _ = iter.next();
-    }
-    #[test] const fn test_const_iters () {
-        let iter = crate::SourceConstIter::new(&":foo :bar");
-        let _ = iter.next();
-    }
-    #[test] fn test_num () {
-        let _digit  = to_digit('0');
-        let _digit  = to_digit('x');
-        let _number = to_number(&"123");
-        let _number = to_number(&"12asdf3");
-    }
-    //proptest! {
-        //#[test] fn proptest_source_iter (
-            //source in "\\PC*"
-        //) {
-            //let mut iter = crate::SourceIter::new(&source);
-            ////let _ = iter.next();
-        //}
-        //#[test] fn proptest_token_iter (
-            //source in "\\PC*"
-        //) {
-            //let mut iter = crate::TokenIter::new(&source);
-            ////let _ = iter.next();
-        //}
-    //}
-}
-
-//#[cfg(test)] mod test_token_prop {
-    //use crate::{CstToken, CstMeta, Value::*};
-    //use proptest::prelude::*;
-    //proptest! {
-        //#[test] fn test_token_prop (
-            //source in "\\PC*",
-            //start  in usize::MIN..usize::MAX,
-            //length in usize::MIN..usize::MAX,
-        //) {
-            //let token = CstToken(Nil, CstMeta { source: &source, start, length });
-            //let _ = token.slice();
-        //}
-    //}
-//}
-
-#[cfg(test)] #[test] fn test_token () -> Result<(), Box<dyn std::error::Error>> {
-    use crate::Value::*;
-    let source = ":f00";
-    let mut token = CstToken(Sym(":"), CstMeta { source, start: 0, length: 1 });
-    token = token.grow_sym();
-    assert_eq!(token, CstToken(Sym(":f"), CstMeta { source, start: 0, length: 2, })); 
-    token = token.grow_sym();
-    assert_eq!(token, CstToken(Sym(":f0"), CstMeta { source, start: 0, length: 3, })); 
-    token = token.grow_sym();
-    assert_eq!(token, CstToken(Sym(":f00"), CstMeta { source, start: 0, length: 4, })); 
-
-    assert_eq!(None,
-        SourceIter::new("").next());
-    assert_eq!(None,
-        SourceIter::new("  \n    \r   \t  ").next());
-    assert_eq!(&Num(7),
-        SourceIter::new("7").next().unwrap().0.value());
-    assert_eq!(&Num(100),
-        SourceIter::new("     100         ").next().unwrap().0.value());
-    assert_eq!(&Err(Unexpected('a')),
-        SourceIter::new("     9a         ").next().unwrap().0.value());
-    assert_eq!(&Sym(":123foo"),
-        SourceIter::new("     :123foo      ").next().unwrap().0.value());
-    assert_eq!(&Sym("@bar456"),
-        SourceIter::new(" \r\r\r\n\n\n@bar456\t\t\t\t\t\t").next().unwrap().0.value());
-    assert_eq!(&Key("foo123"),
-        SourceIter::new("foo123").next().unwrap().0.value());
-    assert_eq!(&Key("foo/bar"),
-        SourceIter::new("foo/bar").next().unwrap().0.value());
-    assert_eq!(&Str("foo/bar"),
-        SourceIter::new("\"foo/bar\"").next().unwrap().0.value());
-    assert_eq!(&Str("foo/bar"),
-        SourceIter::new("    \"foo/bar\"    ").next().unwrap().0.value());
-
-    Ok(())
-}
-
-//#[cfg(test)] #[test] fn test_examples () -> Result<(), DslError> {
-    //// Let's pretend to render some view.
-    //let source = include_str!("../../tek/src/view_arranger.edn");
-    //// The token iterator allows you to get the tokens represented by the source text.
-    //let mut view = TokenIter(source);
-    //// The token iterator wraps a const token+source iterator.
-    //assert_eq!(view.0.0, source);
-    //let mut expr = view.peek();
-    //assert_eq!(view.0.0, source);
-    //assert_eq!(expr, Some(Token {
-        //source, start: 0, length: source.len() - 1, value: Exp(0, SourceIter::new(&source[1..]))
-    //}));
-    ////panic!("{view:?}");
-    ////panic!("{:#?}", expr);
-    ////for example in [
-        ////include_str!("../../tui/examples/edn01.edn"),
-        ////include_str!("../../tui/examples/edn02.edn"),
-    ////] {
-        //////let items = Dsl::read_all(example)?;
-        //////panic!("{layout:?}");
-        //////let content = <dyn ViewContext<::tengri_engine::tui::Tui>>::from(&layout);
-    ////}
-    //Ok(())
-//}
diff --git a/dsl/src/dsl_token.rs b/dsl/src/dsl_token.rs
deleted file mode 100644
index 4411e7e..0000000
--- a/dsl/src/dsl_token.rs
+++ /dev/null
@@ -1,25 +0,0 @@
-use crate::*;
-
-#[derive(PartialEq, Clone, Default, Debug)]
-pub struct Token<
-    V: PartialEq + Clone + Default + Debug,
-    M: PartialEq + Clone + Default + Debug,
->(pub V, pub M);
-
-pub trait DslToken: PartialEq + Clone + Default + Debug {
-    type Value: DslValue;
-    type Meta: Clone + Default + Debug;
-    fn value (&self) -> &Self::Value;
-    fn meta (&self) -> &Self::Meta;
-}
-
-impl<V: DslValue, M: PartialEq + Clone + Default + Debug> DslToken for Token<V, M> {
-    type Value = V;
-    type Meta  = M;
-    fn value (&self) -> &Self::Value { 
-        &self.0
-    }
-    fn meta (&self) -> &Self::Meta {
-        &self.1
-    }
-}
diff --git a/dsl/src/dsl_value.rs b/dsl/src/dsl_value.rs
deleted file mode 100644
index 25d7ec5..0000000
--- a/dsl/src/dsl_value.rs
+++ /dev/null
@@ -1,116 +0,0 @@
-use crate::*;
-
-/// Thing that may construct itself from state and [DslValue].
-pub trait Dsl<State>: Sized {
-    fn try_provide (state: &State, value: impl DslValue) -> Perhaps<Self>;
-    fn provide (
-        state: &State,
-        value: impl DslValue,
-        error: impl Fn()->Box<dyn std::error::Error>
-    ) -> Usually<Self> {
-        match Self::try_provide(state, value)? {
-            Some(value) => Ok(value),
-            _ => Err(error())
-        }
-    }
-}
-
-pub trait DslValue: PartialEq + Clone + Default + Debug {
-    type Str: AsRef<str> + PartialEq + Clone + Default + Debug;
-    type Exp: PartialEq + Clone + Default + Debug;
-    fn value (&self) -> &Value<Self::Str, Self::Exp>;
-    fn nil (&self) -> bool {
-        matches!(self.value(), Value::Nil)
-    }
-    fn err (&self) -> Option<&DslError> {
-        if let Value::Err(e) = self.value() { Some(e) } else { None }
-    }
-    fn num (&self) -> Option<usize> {
-        if let Value::Num(n) = self.value() { Some(*n) } else { None }
-    }
-    fn sym (&self) -> Option<&str> {
-        if let Value::Sym(s) = self.value() { Some(s.as_ref()) } else { None }
-    }
-    fn key (&self) -> Option<&str> {
-        if let Value::Key(k) = self.value() { Some(k.as_ref()) } else { None }
-    }
-    fn str (&self) -> Option<&str> {
-        if let Value::Str(s) = self.value() { Some(s.as_ref()) } else { None }
-    }
-    fn exp (&self) -> Option<&Self::Exp> {
-        if let Value::Exp(_, x) = self.value() { Some(x) } else { None }
-    }
-    fn exp_depth (&self) -> Option<usize>   { None } // TODO
-    fn exp_head  (&self) -> Option<&Self>   { None } // TODO
-    fn exp_tail  (&self) -> Option<&[Self]> { None } // TODO
-}
-
-impl<
-    Str: AsRef<str> + PartialEq + Clone + Default + Debug,
-    Exp: PartialEq + Clone + Default + Debug,
-> DslValue for Value<Str, Exp> {
-    type Str = Str;
-    type Exp = Exp;
-    fn value (&self) -> &Value<Str, Exp> {
-        self
-    }
-}
-
-pub enum Value<S, X> {
-    Nil,
-    Err(DslError),
-    Num(usize),
-    Sym(S),
-    Key(S),
-    Str(S),
-    Exp(usize, X),
-}
-
-impl<S, X> Default for Value<S, X> {
-    fn default () -> Self { Self:: Nil }
-}
-
-impl<S: PartialEq, X: PartialEq,> PartialEq for Value<S, X> {
-    fn eq (&self, other: &Self) -> bool {
-        use Value::*;
-        match (self, other) {
-            (Nil,         Nil)                                 => true,
-            (Err(e1),     Err(e2))     if e1 == e2             => true,
-            (Num(n1),     Num(n2))     if n1 == n2             => true,
-            (Sym(s1),     Sym(s2))     if s1 == s2             => true,
-            (Key(s1),     Key(s2))     if s1 == s2             => true,
-            (Str(s1),     Str(s2))     if s1 == s2             => true,
-            (Exp(d1, e1), Exp(d2, e2)) if d1 == d2 && e1 == e2 => true,
-            _ => false
-        }
-    }
-}
-
-impl<S: Clone, X: Clone,> Clone for Value<S, X> {
-    fn clone (&self) -> Self {
-        use Value::*;
-        match self {
-            Nil => Nil,
-            Err(e) => Err(e.clone()),
-            Num(n) => Num(*n),
-            Sym(s) => Sym(s.clone()),
-            Key(s) => Key(s.clone()),
-            Str(s) => Str(s.clone()),
-            Exp(d, e) => Exp(*d, e.clone()),
-        }
-    }
-}
-
-impl<S: Copy, X: Copy,> Copy for Value<S, X> {}
-
-impl<S: Debug, X: Debug,> Debug for Value<S, X> {
-    fn fmt (&self, _f: &mut std::fmt::Formatter) -> Result<(), std::fmt::Error> {
-        todo!()
-    }
-}
-
-impl<S: Display, X: Display,> Display for Value<S, X> {
-    fn fmt (&self, _f: &mut std::fmt::Formatter) -> Result<(), std::fmt::Error> {
-        todo!()
-    }
-}
diff --git a/dsl/src/lib.rs b/dsl/src/lib.rs
index 485f09e..02f82a5 100644
--- a/dsl/src/lib.rs
+++ b/dsl/src/lib.rs
@@ -35,17 +35,118 @@
 #![feature(type_alias_impl_trait)]
 #![feature(impl_trait_in_fn_trait_return)]
 pub(crate) use ::tengri_core::*;
-pub(crate) use std::fmt::Debug;
-pub(crate) use std::fmt::{Display, Formatter, Error as FormatError};
+pub(crate) use std::fmt::{Debug, Display, Formatter, Error as FormatError};
+pub(crate) use std::sync::Arc;
+pub(crate) use std::collections::VecDeque;
 pub(crate) use konst::iter::{ConstIntoIter, IsIteratorKind};
 pub(crate) use konst::string::{split_at, str_range, char_indices};
 pub(crate) use thiserror::Error;
 pub(crate) use self::DslError::*;
-mod dsl_ast;     pub use self::dsl_ast::*;
-mod dsl_cst;     pub use self::dsl_cst::*;
-mod dsl_display; //pub use self::dsl_display::*;
-mod dsl_error;   pub use self::dsl_error::*;
-mod dsl_iter;    pub use self::dsl_iter::*;
-mod dsl_token;   pub use self::dsl_token::*;
-mod dsl_value;   pub use self::dsl_value::*;
-#[cfg(test)] mod dsl_test;
+
+mod dsl; pub use self::dsl::*;
+mod ast; pub use self::ast::*;
+mod cst; pub use self::cst::*;
+
+#[cfg(test)] mod test_token_iter {
+    use crate::*;
+    //use proptest::prelude::*;
+    #[test] fn test_iters () {
+        let mut iter = crate::SourceIter::new(&":foo :bar");
+        let _ = iter.next();
+    }
+    #[test] const fn test_const_iters () {
+        let iter = crate::SourceConstIter::new(&":foo :bar");
+        let _ = iter.next();
+    }
+    #[test] fn test_num () {
+        let _digit  = to_digit('0');
+        let _digit  = to_digit('x');
+        let _number = to_number(&"123");
+        let _number = to_number(&"12asdf3");
+    }
+    //proptest! {
+        //#[test] fn proptest_source_iter (
+            //source in "\\PC*"
+        //) {
+            //let mut iter = crate::SourceIter::new(&source);
+            ////let _ = iter.next();
+        //}
+        //#[test] fn proptest_token_iter (
+            //source in "\\PC*"
+        //) {
+            //let mut iter = crate::TokenIter::new(&source);
+            ////let _ = iter.next();
+        //}
+    //}
+}
+
+//#[cfg(test)] mod test_token_prop {
+    //use crate::{Cst, CstMeta, Value::*};
+    //use proptest::prelude::*;
+    //proptest! {
+        //#[test] fn test_token_prop (
+            //source in "\\PC*",
+            //start  in usize::MIN..usize::MAX,
+            //length in usize::MIN..usize::MAX,
+        //) {
+            //let token = Cst(Nil, CstMeta { source: &source, start, length });
+            //let _ = token.slice();
+        //}
+    //}
+//}
+
+#[cfg(test)] #[test] fn test_token () -> Result<(), Box<dyn std::error::Error>> {
+    use crate::DslValue::*;
+    let source = ":f00";
+    let mut token = Cst::new(source, 0, 1, Sym(":"));
+    token = token.grow_sym();
+    assert_eq!(token, Cst::new(source, 0, 2, Sym(":f")));
+    token = token.grow_sym();
+    assert_eq!(token, Cst::new(source, 0, 3, Sym(":f0")));
+    token = token.grow_sym();
+    assert_eq!(token, Cst::new(source, 0, 4, Sym(":f00")));
+
+    assert_eq!(None, SourceIter::new("").next());
+    assert_eq!(None, SourceIter::new("  \n    \r   \t  ").next());
+
+    assert_eq!(Err(Unexpected('a')), SourceIter::new("     9a         ").next().unwrap().value());
+
+    assert_eq!(Num(7),   SourceIter::new("7").next().unwrap().value());
+    assert_eq!(Num(100), SourceIter::new("     100       ").next().unwrap().value());
+
+    assert_eq!(Sym(":123foo"), SourceIter::new("     :123foo      ").next().unwrap().value());
+    assert_eq!(Sym("@bar456"), SourceIter::new(" \r\r\n\n@bar456\t\t\t").next().unwrap().value());
+
+    assert_eq!(Key("foo123"),  SourceIter::new("foo123").next().unwrap().value());
+    assert_eq!(Key("foo/bar"), SourceIter::new("foo/bar").next().unwrap().value());
+
+    assert_eq!(Str("foo/bar"), SourceIter::new("\"foo/bar\"").next().unwrap().value());
+    assert_eq!(Str("foo/bar"), SourceIter::new("    \"foo/bar\"    ").next().unwrap().value());
+
+    Ok(())
+}
+
+//#[cfg(test)] #[test] fn test_examples () -> Result<(), DslError> {
+    //// Let's pretend to render some view.
+    //let source = include_str!("../../tek/src/view_arranger.edn");
+    //// The token iterator allows you to get the tokens represented by the source text.
+    //let mut view = TokenIter(source);
+    //// The token iterator wraps a const token+source iterator.
+    //assert_eq!(view.0.0, source);
+    //let mut expr = view.peek();
+    //assert_eq!(view.0.0, source);
+    //assert_eq!(expr, Some(Token {
+        //source, start: 0, length: source.len() - 1, value: Exp(0, SourceIter::new(&source[1..]))
+    //}));
+    ////panic!("{view:?}");
+    ////panic!("{:#?}", expr);
+    ////for example in [
+        ////include_str!("../../tui/examples/edn01.edn"),
+        ////include_str!("../../tui/examples/edn02.edn"),
+    ////] {
+        //////let items = Dsl::read_all(example)?;
+        //////panic!("{layout:?}");
+        //////let content = <dyn ViewContext<::tengri_engine::tui::Tui>>::from(&layout);
+    ////}
+    //Ok(())
+//}
diff --git a/input/src/input_dsl.rs b/input/src/input_dsl.rs
index 76f3c3b..0cdb7c0 100644
--- a/input/src/input_dsl.rs
+++ b/input/src/input_dsl.rs
@@ -42,7 +42,7 @@ impl<'a, S: Eval<Ast, bool> + Eval<Ast, O>, I: Eval<Ast, bool>, O: Command<S>> E
                 matches = state.eval(condition.clone(), ||"input: no condition")?;
             }
             if matches
-            && let Some(exp) = bindings.exp()
+            && let Some(exp) = bindings.0.as_exp()
             && let Some(ast) = exp.peek()
             && input.eval(ast.clone(), ||"InputLayers: input.eval(binding) failed")?
             && let Some(command) = state.try_eval(ast)? {
diff --git a/output/src/ops_dsl.rs b/output/src/ops_dsl.rs
index 94ded3f..be722ac 100644
--- a/output/src/ops_dsl.rs
+++ b/output/src/ops_dsl.rs
@@ -1,91 +1,65 @@
 use crate::*;
-use Value::*;
+use DslValue::*;
 
-impl<S, A> Dsl<S> for When<A> where S: Eval<Ast, bool> + Eval<Ast, A> {
-    fn try_provide (state: &S, source: impl DslValue) -> Perhaps<Self> {
-        if let Exp(_, mut exp) = source.value()
-        && let Some(Ast(Key(id))) = exp.peek() && *id == *"when" {
-            let _ = exp.next();
-            return Ok(Some(Self(
-                state.eval(exp.next().unwrap(), ||"when: expected condition")?,
-                state.eval(exp.next().unwrap(), ||"when: expected content")?,
-            )))
-        }
+fn exp_match <S: DslValue, E: DslIter, T> (source: S, namespace: &str, cb: impl Fn(&str, E)) -> Perhaps<T> {
+    if let Some(exp) = source.exp()
+    && let Some(Key(key)) = exp.next()
+    && key.starts_with(namespace) {
+        cb(key.split_at(namespace.len()).1, exp)
+    } else {
         Ok(None)
     }
 }
 
-impl<S, A, B> Dsl<S> for Either<A, B> where S: Eval<Ast, bool> + Eval<Ast, A> + Eval<Ast, B> {
-    fn try_provide (state: &S, source: impl DslValue) -> Perhaps<Self> {
-        if let Exp(_, mut exp) = source.value()
-        && let Some(Ast(Key(id))) = exp.peek() && *id == *"either" {
-            let _ = exp.next();
-            return Ok(Some(Self(
-                state.eval(exp.next().unwrap(), ||"either: expected condition")?,
-                state.eval(exp.next().unwrap(), ||"either: expected content 1")?,
-                state.eval(exp.next().unwrap(), ||"either: expected content 2")?,
-            )))
-        }
-        Ok(None)
+impl<S, A> FromDsl<S> for When<A> where S: Eval<Ast, bool> + Eval<Ast, A> {
+    fn try_provide (state: &S, source: DslValue<impl DslStr, impl DslExp>) -> Perhaps<Self> {
+        exp_match::<S, Self>(source, "when", |_, tail|Ok(Some(Self(
+            tail.eval(0, ||"no condition")?,
+            tail.eval(1, ||"no content")?,
+        ))))
     }
 }
 
-impl<S, A> Dsl<S> for Align<A> where S: Eval<Option<Ast>, A> {
-    fn try_provide (state: &S, source: impl DslValue) -> Perhaps<Self> {
-        if let Exp(_, source) = source.value() {
-            let mut rest = source.clone();
-            return Ok(Some(match rest.next().as_ref().and_then(|x|x.key()) {
-                Some("align/c")  => Self::c(state.eval(rest.next(),  ||"align/c: expected content")?),
-                Some("align/x")  => Self::x(state.eval(rest.next(),  ||"align/x: expected content")?),
-                Some("align/y")  => Self::y(state.eval(rest.next(),  ||"align/y: expected content")?),
-                Some("align/n")  => Self::n(state.eval(rest.next(),  ||"align/n: expected content")?),
-                Some("align/s")  => Self::s(state.eval(rest.next(),  ||"align/s: expected content")?),
-                Some("align/e")  => Self::e(state.eval(rest.next(),  ||"align/e: expected content")?),
-                Some("align/w")  => Self::w(state.eval(rest.next(),  ||"align/w: expected content")?),
-                Some("align/nw") => Self::nw(state.eval(rest.next(), ||"align/nw: expected content")?),
-                Some("align/ne") => Self::ne(state.eval(rest.next(), ||"align/ne: expected content")?),
-                Some("align/sw") => Self::sw(state.eval(rest.next(), ||"align/sw: expected content")?),
-                Some("align/se") => Self::se(state.eval(rest.next(), ||"align/se: expected content")?),
-                _ => return Ok(None),
-            }))
-        }
-        Ok(None)
+impl<S, A, B> FromDsl<S> for Either<A, B> where S: Eval<Ast, bool> + Eval<Ast, A> + Eval<Ast, B> {
+    fn try_provide (state: &S, source: DslValue<impl DslStr, impl DslExp>) -> Perhaps<Self> {
+        exp_match::<S, Self>(source, "either", |_, tail|Ok(Some(Self(
+            tail.eval(0, ||"no condition")?,
+            tail.eval(1, ||"no content 1")?,
+            tail.eval(2, ||"no content 2")?,
+        ))))
     }
 }
 
-impl<S, A, B> Dsl<S> for Bsp<A, B> where S: Eval<Option<Ast>, A> + Eval<Option<Ast>, B> {
-    fn try_provide (state: &S, source: impl DslValue) -> Perhaps<Self> {
-        if let Exp(_, exp) = source.value() {
-            let mut rest = exp.clone();
-            return Ok(Some(match rest.next().as_ref().and_then(|x|x.key()) {
-                Some("bsp/n") => Self::n(
-                    state.eval(rest.next(), ||"bsp/n: expected content 1")?,
-                    state.eval(rest.next(), ||"bsp/n: expected content 2")?,
-                ),
-                Some("bsp/s") => Self::s(
-                    state.eval(rest.next(), ||"bsp/s: expected content 1")?,
-                    state.eval(rest.next(), ||"bsp/s: expected content 2")?,
-                ),
-                Some("bsp/e") => Self::e(
-                    state.eval(rest.next(), ||"bsp/e: expected content 1")?,
-                    state.eval(rest.next(), ||"bsp/e: expected content 2")?,
-                ),
-                Some("bsp/w") => Self::w(
-                    state.eval(rest.next(), ||"bsp/w: expected content 1")?,
-                    state.eval(rest.next(), ||"bsp/w: expected content 2")?,
-                ),
-                Some("bsp/a") => Self::a(
-                    state.eval(rest.next(), ||"bsp/a: expected content 1")?,
-                    state.eval(rest.next(), ||"bsp/a: expected content 2")?,
-                ),
-                Some("bsp/b") => Self::b(
-                    state.eval(rest.next(), ||"bsp/b: expected content 1")?,
-                    state.eval(rest.next(), ||"bsp/b: expected content 2")?,
-                ),
-                _ => return Ok(None),
-            }))
-        }
-        Ok(None)
+impl<S, A> FromDsl<S> for Align<A> where S: Eval<Option<Ast>, A> {
+    fn try_provide (state: &S, source: DslValue<impl DslStr, impl DslExp>) -> Perhaps<Self> {
+        exp_match::<S, Self>(source, "align/", |head, tail|Ok(Some(match head {
+            "c"  => Self::c(tail.eval(0,  ||"no content")?),
+            "x"  => Self::x(tail.eval(0,  ||"no content")?),
+            "y"  => Self::y(tail.eval(0,  ||"no content")?),
+            "n"  => Self::n(tail.eval(0,  ||"no content")?),
+            "s"  => Self::s(tail.eval(0,  ||"no content")?),
+            "e"  => Self::e(tail.eval(0,  ||"no content")?),
+            "w"  => Self::w(tail.eval(0,  ||"no content")?),
+            "nw" => Self::nw(tail.eval(0, ||"no content")?),
+            "ne" => Self::ne(tail.eval(0, ||"no content")?),
+            "sw" => Self::sw(tail.eval(0, ||"no content")?),
+            "se" => Self::se(tail.eval(0, ||"no content")?),
+            _ => return Err("invalid align variant")
+        })))
+    }
+}
+
+impl<S, A, B> FromDsl<S> for Bsp<A, B> where S: Eval<Option<Ast>, A> + Eval<Option<Ast>, B> {
+    fn try_provide (state: &S, source: DslValue<impl DslStr, impl DslExp>) -> Perhaps<Self> {
+        exp_match::<S, Self>(source, "bsp/", |head, tail|Ok(Some(match head {
+            "n" => Self::n(tail.eval(0, ||"no content 1"), tail.eval(1, ||"no content 2")),
+            "s" => Self::s(tail.eval(0, ||"no content 1"), tail.eval(1, ||"no content 2")),
+            "e" => Self::e(tail.eval(0, ||"no content 1"), tail.eval(1, ||"no content 2")),
+            "w" => Self::w(tail.eval(0, ||"no content 1"), tail.eval(1, ||"no content 2")),
+            "a" => Self::a(tail.eval(0, ||"no content 1"), tail.eval(1, ||"no content 2")),
+            "b" => Self::b(tail.eval(0, ||"no content 1"), tail.eval(1, ||"no content 2")),
+            _ => return Ok(None),
+        })))
     }
 }
 
@@ -124,3 +98,77 @@ impl<S, A, B> Dsl<S> for Bsp<A, B> where S: Eval<Option<Ast>, A> + Eval<Option<A
             //} else {
                 //None
             //}));
+        //if let Exp(_, exp) = source.value() {
+            //let mut rest = exp.clone();
+            //return Ok(Some(match rest.next().as_ref().and_then(|x|x.key()) {
+                //Some("bsp/n") => Self::n(
+                    //state.eval(rest.next(), ||"bsp/n: no content 1")?,
+                    //state.eval(rest.next(), ||"bsp/n: no content 2")?,
+                //),
+                //Some("bsp/s") => Self::s(
+                    //state.eval(rest.next(), ||"bsp/s: no content 1")?,
+                    //state.eval(rest.next(), ||"bsp/s: no content 2")?,
+                //),
+                //Some("bsp/e") => Self::e(
+                    //state.eval(rest.next(), ||"bsp/e: no content 1")?,
+                    //state.eval(rest.next(), ||"bsp/e: no content 2")?,
+                //),
+                //Some("bsp/w") => Self::w(
+                    //state.eval(rest.next(), ||"bsp/w: no content 1")?,
+                    //state.eval(rest.next(), ||"bsp/w: no content 2")?,
+                //),
+                //Some("bsp/a") => Self::a(
+                    //state.eval(rest.next(), ||"bsp/a: no content 1")?,
+                    //state.eval(rest.next(), ||"bsp/a: no content 2")?,
+                //),
+                //Some("bsp/b") => Self::b(
+                    //state.eval(rest.next(), ||"bsp/b: no content 1")?,
+                    //state.eval(rest.next(), ||"bsp/b: no content 2")?,
+                //),
+                //_ => return Ok(None),
+            //}))
+        //}
+        //Ok(None)
+        //if let Exp(_, source) = source.value() {
+            //let mut rest = source.clone();
+            //return Ok(Some(match rest.next().as_ref().and_then(|x|x.key()) {
+                //Some("align/c")  => Self::c(state.eval(rest.next(),  ||"align/c: no content")?),
+                //Some("align/x")  => Self::x(state.eval(rest.next(),  ||"align/x: no content")?),
+                //Some("align/y")  => Self::y(state.eval(rest.next(),  ||"align/y: no content")?),
+                //Some("align/n")  => Self::n(state.eval(rest.next(),  ||"align/n: no content")?),
+                //Some("align/s")  => Self::s(state.eval(rest.next(),  ||"align/s: no content")?),
+                //Some("align/e")  => Self::e(state.eval(rest.next(),  ||"align/e: no content")?),
+                //Some("align/w")  => Self::w(state.eval(rest.next(),  ||"align/w: no content")?),
+                //Some("align/nw") => Self::nw(state.eval(rest.next(), ||"align/nw: no content")?),
+                //Some("align/ne") => Self::ne(state.eval(rest.next(), ||"align/ne: no content")?),
+                //Some("align/sw") => Self::sw(state.eval(rest.next(), ||"align/sw: no content")?),
+                //Some("align/se") => Self::se(state.eval(rest.next(), ||"align/se: no content")?),
+                //_ => return Ok(None),
+            //}))
+        //}
+        //Ok(None)
+        //Ok(match source.exp_head().and_then(|e|e.key()) {
+            //Some("either") => Some(Self(
+                //source.exp_tail().and_then(|t|t.get(0)).map(|x|state.eval(x, ||"when: no condition"))?,
+                //source.exp_tail().and_then(|t|t.get(1)).map(|x|state.eval(x, ||"when: no content 1"))?,
+                //source.exp_tail().and_then(|t|t.get(2)).map(|x|state.eval(x, ||"when: no content 2"))?,
+            //)),
+            //_ => None
+        //})
+        //if let Exp(_, mut exp) = source.value()
+        //&& let Some(Ast(Key(id))) = exp.peek() && *id == *"either" {
+            //let _ = exp.next();
+            //return Ok(Some(Self(
+                //state.eval(exp.next().unwrap(), ||"either: no condition")?,
+                //state.eval(exp.next().unwrap(), ||"either: no content 1")?,
+                //state.eval(exp.next().unwrap(), ||"either: no content 2")?,
+            //)))
+        //}
+        //Ok(None)
+        //Ok(match source.exp_head().and_then(|e|e.key()) {
+            //Some("when") => Some(Self(
+                //source.exp_tail().and_then(|t|t.get(0)).map(|x|state.eval(x, ||"when: no condition"))?,
+                //source.exp_tail().and_then(|t|t.get(1)).map(|x|state.eval(x, ||"when: no content"))?,
+            //)),
+            //_ => None
+        //})