diff --git a/dsl/src/dsl_ast.rs b/dsl/src/dsl_ast.rs
index 15c4374..24a0f6f 100644
--- a/dsl/src/dsl_ast.rs
+++ b/dsl/src/dsl_ast.rs
@@ -1,70 +1,46 @@
 use crate::*;
 use std::sync::Arc;
-use std::fmt::{Debug, Display, Formatter};
+use std::borrow::Cow;
 
-#[derive(Clone, Default, Debug)]
-pub enum Ast {
-    #[default] Nil,
-    Err(DslError),
-    Num(usize),
-    Sym(Arc<str>),
-    Key(Arc<str>),
-    Str(Arc<str>),
-    Exp(Arc<Box<dyn AstIter>>),
-}
+#[derive(Debug, Clone, Default, PartialEq)]
+pub struct Ast(pub Value<Arc<str>, AstIter>);
 
-/// Emits tokens.
-pub trait AstIter: Debug {
-    fn peek (&self) -> Option<Ast>;
-    fn next (&mut self) -> Option<Ast>;
-    fn rest (self) -> Option<Box<dyn AstIter>>;
-}
-
-/// A [Cst] can be used as an [Ast].
-impl<'source: 'static> AstIter for Cst<'source> {
-    fn peek (&self) -> Option<Ast> {
-        Cst::peek(self).map(|token|token.value.into())
-    }
-    fn next (&mut self) -> Option<Ast> {
-        Iterator::next(self).map(|token|token.value.into())
-    }
-    fn rest (self) -> Option<Box<dyn AstIter>> {
-        self.peek().is_some().then(||Box::new(self) as Box<dyn AstIter>)
+impl<'source> From<CstToken<'source>> for Ast {
+    fn from (token: CstToken<'source>) -> Self {
+        token.value().into()
     }
 }
 
-impl std::fmt::Display for Ast {
-    fn fmt (&self, out: &mut Formatter) -> Result<(), std::fmt::Error> {
-        use Ast::*;
-        write!(out, "{}", match self {
-            Nil => String::new(),
-            Err(e) => format!("[error: {e}]"),
-            Num(n) => format!("{n}"),
-            Sym(s) => format!("{s}"),
-            Key(s) => format!("{s}"),
-            Str(s) => format!("{s}"),
-            Exp(e) => format!("{e:?}"),
+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<'source: 'static> From<CstValue<'source>> for Ast {
-    fn from (other: CstValue<'source>) -> Self {
-        use CstValue::*;
-        match other {
-            Nil => Self::Nil,
-            Err(e) => Self::Err(e),
-            Num(u) => Self::Num(u),
-            Sym(s) => Self::Sym(s.into()),
-            Key(s) => Self::Key(s.into()),
-            Str(s) => Self::Str(s.into()),
-            Exp(_, s) => Self::Exp(Arc::new(s.into())),
-        }
+impl DslValue for Ast {
+    type Str = Arc<str>;
+    type Exp = AstIter;
+    fn value (&self) -> &Value<Arc<str>, AstIter> {
+        self.0.value()
     }
 }
 
-impl<'source: 'static> Into<Box<dyn AstIter>> for Cst<'source> {
-    fn into (self) -> Box<dyn AstIter> {
-        Box::new(self)
+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
index a45e77a..7757a70 100644
--- a/dsl/src/dsl_cst.rs
+++ b/dsl/src/dsl_cst.rs
@@ -1,110 +1,117 @@
 use crate::*;
 
-/// Provides a native [Iterator] API over [CstConstIter],
-/// emitting [CstToken] items.
-///
-/// [Cst::next] returns just the [CstToken] and mutates `self`,
-/// instead of returning an updated version of the struct as [CstConstIter::next] does.
-#[derive(Copy, Clone, Debug, Default, PartialEq)]
-pub struct Cst<'a>(pub CstConstIter<'a>);
+/// CST stores strings as source references and expressions as new [SourceIter] instances.
+pub type CstValue<'source> = Value<&'source str, SourceIter<'source>>;
 
-/// Owns a reference to the source text.
-/// [CstConstIter::next] emits subsequent pairs of:
-///   * a [CstToken] and
-///   * the source text remaining
-///     * [ ] TODO: maybe [CstConstIter::next] should wrap the remaining source in `Self` ?
-#[derive(Copy, Clone, Debug, Default, PartialEq)]
-pub struct CstConstIter<'a>(pub &'a str);
-
-/// A CST token, with reference to the source slice.
-#[derive(Debug, Copy, Clone, Default, PartialEq)] pub struct CstToken<'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,
-    pub value:  CstValue<'source>,
 }
 
-/// The meaning of a CST token. Strip the source from this to get an [AstValue].
-#[derive(Debug, Copy, Clone, Default, PartialEq)] pub enum CstValue<'source> {
-    #[default] Nil,
-    Err(DslError),
-    Num(usize),
-    Sym(&'source str),
-    Key(&'source str),
-    Str(&'source str),
-    Exp(usize, Cst<'source>),
-}
+/// Token sharing memory with source reference.
+#[derive(Debug, Copy, Clone, Default, PartialEq)]
+pub struct CstToken<'source>(pub CstValue<'source>, pub CstMeta<'source>);
 
-impl<'a> Cst<'a> {
-    pub const fn new (source: &'a str) -> Self {
-        Self(CstConstIter::new(source))
+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 peek (&self) -> Option<CstToken<'a>> {
-        self.0.peek()
+    pub const fn end (&self) -> usize {
+        self.1.start.saturating_add(self.1.length)
     }
-}
-
-impl<'a> Iterator for Cst<'a> {
-    type Item = CstToken<'a>;
-    fn next (&mut self) -> Option<CstToken<'a>> {
-        self.0.next().map(|(item, rest)|{self.0 = rest; item})
+    pub const fn slice (&'source self) -> &'source str {
+        self.slice_source(self.1.source)
     }
-}
-
-impl<'a> From<&'a str> for Cst<'a> {
-    fn from (source: &'a str) -> Self{
-        Self(CstConstIter(source))
+    pub const fn slice_source <'range> (&'source self, source: &'range str) -> &'range str {
+        str_range(source, self.1.start, self.end())
     }
-}
-
-impl<'a> From<CstConstIter<'a>> for Cst<'a> {
-    fn from (source: CstConstIter<'a>) -> Self{
-        Self(source)
+    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())
     }
-}
-
-/// 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> { $expr }
+    pub const fn with_value (self, value: CstValue<'source>) -> Self {
+        Self(value, self.1)
+    }
+    pub const fn value (&self) -> CstValue<'source> {
+        self.0
+    }
+    pub const fn error (self, error: DslError) -> Self {
+        Self(Value::Err(error), self.1)
+    }
+    pub const fn grow (self) -> Self {
+        Self(self.0, CstMeta { length: self.1.length.saturating_add(1), ..self.1 })
+    }
+    pub const fn grow_num (self, m: usize, c: char) -> Self {
+        use Value::*;
+        match to_digit(c) {
+            Result::Ok(n)  => Self(Num(10*m+n), self.grow().1),
+            Result::Err(e) => Self(Err(e), self.grow().1),
         }
-        impl$(<$l>)? ConstIntoIter for $Struct {
-            type Kind = IsIteratorKind;
-            type Item = $Item;
-            type IntoIter = Self;
+    }
+    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!()
         }
     }
 }
 
-const_iter!(<'a>|self: CstConstIter<'a>| => CstToken<'a> => self.next_mut().map(|(result, _)|result));
-
-impl<'a> From<&'a str> for CstConstIter<'a> {
-    fn from (source: &'a str) -> Self{
-        Self::new(source)
-    }
+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)
 }
 
-impl<'a> CstConstIter<'a> {
-    pub const fn new (source: &'a 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<'a>, Self)> {
-        Self::next_mut(&mut self)
-    }
-    pub const fn peek (&self) -> Option<CstToken<'a>> {
-        peek_src(self.0)
-    }
-    pub const fn next_mut (&mut self) -> Option<(CstToken<'a>, Self)> {
-        match self.peek() {
-            Some(token) => Some((token, self.chomp(token.end()))),
-            None => None
-        }
-    }
+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.
@@ -119,7 +126,7 @@ impl<'a> CstConstIter<'a> {
 }
 
 pub const fn peek_src <'a> (source: &'a str) -> Option<CstToken<'a>> {
-    use CstValue::*;
+    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),
@@ -127,7 +134,7 @@ pub const fn peek_src <'a> (source: &'a str) -> Option<CstToken<'a>> {
             ' '|'\n'|'\r'|'\t' =>
                 token.grow(),
             '(' =>
-                CstToken::new(source, start, 1, Exp(1, Cst::new(str_range(source, start, start + 1)))),
+                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))),
             ':'|'@' =>
@@ -136,8 +143,8 @@ pub const fn peek_src <'a> (source: &'a str) -> Option<CstToken<'a>> {
                 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) => CstValue::Num(c),
-                    Result::Err(e) => CstValue::Err(e)
+                    Ok(c) => Value::Num(c),
+                    Result::Err(e) => Value::Err(e)
                 }),
             _ => token.error(Unexpected(c))
         },
@@ -174,118 +181,3 @@ pub const fn peek_src <'a> (source: &'a str) -> Option<CstToken<'a>> {
         _ => Some(token),
     }
 }
-
-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))
-    })
-}
-
-impl<'source> CstToken<'source> {
-    pub const fn new (
-        source: &'source str, start: usize, length: usize, value: CstValue<'source>
-    ) -> Self {
-        Self { source, start, length, value }
-    }
-    pub const fn end (&self) -> usize {
-        self.start.saturating_add(self.length)
-    }
-    pub const fn slice (&'source self) -> &'source str {
-        self.slice_source(self.source)
-    }
-    pub const fn slice_source <'range> (&'source self, source: &'range str) -> &'range str {
-        str_range(source, self.start, self.end())
-    }
-    pub const fn slice_source_exp <'range> (&'source self, source: &'range str) -> &'range str {
-        str_range(source, self.start.saturating_add(1), self.end())
-    }
-    pub const fn with_value (self, value: CstValue<'source>) -> Self {
-        Self { value, ..self }
-    }
-    pub const fn value (&self) -> CstValue {
-        self.value
-    }
-    pub const fn error (self, error: DslError) -> Self {
-        Self { value: CstValue::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 {
-        use CstValue::*;
-        match to_digit(c) {
-            Ok(n) => Self { value: Num(10*m+n), ..self.grow() },
-            Result::Err(e) => Self { value: Err(e), ..self.grow() },
-        }
-    }
-    pub const fn grow_key (self) -> Self {
-        use CstValue::*;
-        let token = self.grow();
-        token.with_value(Key(token.slice_source(self.source)))
-    }
-    pub const fn grow_sym (self) -> Self {
-        use CstValue::*;
-        let token = self.grow();
-        token.with_value(Sym(token.slice_source(self.source)))
-    }
-    pub const fn grow_str (self) -> Self {
-        use CstValue::*;
-        let token = self.grow();
-        token.with_value(Str(token.slice_source(self.source)))
-    }
-    pub const fn grow_exp (self) -> Self {
-        use CstValue::*;
-        let token = self.grow();
-        if let Exp(depth, _) = token.value {
-            token.with_value(Exp(depth, Cst::new(token.slice_source_exp(self.source))))
-        } else {
-            unreachable!()
-        }
-    }
-    pub const fn grow_in (self) -> Self {
-        let token = self.grow_exp();
-        if let CstValue::Exp(depth, source) = token.value {
-            token.with_value(CstValue::Exp(depth.saturating_add(1), source))
-        } else {
-            unreachable!()
-        }
-    }
-    pub const fn grow_out (self) -> Self {
-        let token = self.grow_exp();
-        if let CstValue::Exp(depth, source) = token.value {
-            if depth > 0 {
-                token.with_value(CstValue::Exp(depth - 1, source))
-            } else {
-                return self.error(Unexpected(')'))
-            }
-        } else {
-            unreachable!()
-        }
-    }
-}
-
-impl<'source> std::fmt::Display for CstValue<'source> {
-    fn fmt (&self, out: &mut std::fmt::Formatter) -> Result<(), std::fmt::Error> {
-        use CstValue::*;
-        write!(out, "{}", match self {
-            Nil    => String::new(),
-            Err(e) => format!("[error: {e}]"),
-            Num(n) => format!("{n}"),
-            Sym(s) => format!("{s}"),
-            Key(s) => format!("{s}"),
-            Str(s) => format!("{s}"),
-            Exp(_, e) => format!("{e:?}"),
-        })
-    }
-}
diff --git a/dsl/src/dsl_display.rs b/dsl/src/dsl_display.rs
new file mode 100644
index 0000000..aaae8c7
--- /dev/null
+++ b/dsl/src/dsl_display.rs
@@ -0,0 +1,30 @@
+use crate::*;
+use std::fmt::{Debug, Display, Formatter, Error as FormatError};
+impl Display for Ast {
+    fn fmt (&self, out: &mut Formatter) -> Result<(), FormatError> {
+        use Value::*;
+        write!(out, "{}", match &self.0 {
+            Nil => String::new(),
+            Err(e) => format!("[error: {e}]"),
+            Num(n) => format!("{n}"),
+            Sym(s) => format!("{s}"),
+            Key(s) => format!("{s}"),
+            Str(s) => format!("{s}"),
+            Exp(_, e) => format!("{e:?}"),
+        })
+    }
+}
+impl<'source> Display for CstValue<'source> {
+    fn fmt (&self, out: &mut Formatter) -> Result<(), FormatError> {
+        use Value::*;
+        write!(out, "{}", match self {
+            Nil    => String::new(),
+            Err(e) => format!("[error: {e}]"),
+            Num(n) => format!("{n}"),
+            Sym(s) => format!("{s}"),
+            Key(s) => format!("{s}"),
+            Str(s) => format!("{s}"),
+            Exp(_, e) => format!("{e:?}"),
+        })
+    }
+}
diff --git a/dsl/src/dsl_domain.rs b/dsl/src/dsl_domain.rs
index 88af556..d8d502e 100644
--- a/dsl/src/dsl_domain.rs
+++ b/dsl/src/dsl_domain.rs
@@ -13,13 +13,25 @@ pub trait Eval<Input, Output> {
     }
 }
 
+//impl<S: Eval<I, O>, I, O> Eval<I, O> for &S {
+    //fn try_eval (&self, input: I) -> Perhaps<O> {
+        //(*self).try_eval(input)
+    //}
+//}
+
+//impl<S: Eval<I, O>, I: Ast, O: Dsl<S>> Eval<I, O> for S {
+    //fn try_eval (&self, input: I) -> Perhaps<O> {
+        //Dsl::try_provide(self, input)
+    //}
+//}
+
 /// May construct [Self] from token stream.
 pub trait Dsl<State>: Sized {
-    fn try_provide (state: State, source: Ast) -> Perhaps<Self>;
+    fn try_provide (state: &State, source: Ast) -> Perhaps<Self>;
     fn provide <E: Into<Box<dyn std::error::Error>>, F: Fn()->E> (
-        state: State, source: Ast, error: F
+        state: &State, source: Ast, error: F
     ) -> Usually<Self> {
-        let next = source.clone();
+        let next = format!("{source:?}");
         if let Some(value) = Self::try_provide(state, source)? {
             Ok(value)
         } else {
@@ -28,10 +40,6 @@ pub trait Dsl<State>: Sized {
     }
 }
 
-impl<T: Dsl<S>, S> Eval<Ast, T> for S {
-    fn try_eval (&self, input: Ast) -> Perhaps<T> { todo!() }
-}
-
 //pub trait Give<'state, 'source, Type> {
     ///// Implement this to be able to [Give] [Type] from the [Cst].
     ///// Advance the stream if returning `Ok<Some<Type>>`.
diff --git a/dsl/src/dsl_iter.rs b/dsl/src/dsl_iter.rs
new file mode 100644
index 0000000..07cfdd7
--- /dev/null
+++ b/dsl/src/dsl_iter.rs
@@ -0,0 +1,130 @@
+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>;
+}
+
+#[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())
+    }
+}
+
+/// 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()
+    }
+}
+
+/// 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)
+    }
+}
+
+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));
diff --git a/dsl/src/dsl_test.rs b/dsl/src/dsl_test.rs
new file mode 100644
index 0000000..4f7c4b2
--- /dev/null
+++ b/dsl/src/dsl_test.rs
@@ -0,0 +1,124 @@
+#[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();
+        let mut iter = crate::TokenIter::new(&":foo :bar");
+        let _ = iter.next();
+    }
+    #[test] const fn test_const_iters () {
+        let mut iter = crate::SourceIter::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 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 = crate::Token {
+                source: &source,
+                start,
+                length,
+                value: crate::Value::Nil
+            };
+            let _ = token.slice();
+        }
+    }
+}
+
+#[cfg(test)] #[test] fn test_token () -> Result<(), Box<dyn std::error::Error>> {
+    let source = ":f00";
+    let mut token = Token { source, start: 0, length: 1, value: Sym(":") };
+    token = token.grow_sym();
+    assert_eq!(token, Token { source, start: 0, length: 2, value: Sym(":f") }); 
+    token = token.grow_sym();
+    assert_eq!(token, Token { source, start: 0, length: 3, value: Sym(":f0") }); 
+    token = token.grow_sym();
+    assert_eq!(token, Token { source, start: 0, length: 4, value: Sym(":f00") }); 
+
+    let src = "";
+    assert_eq!(None, SourceIter(src).next());
+
+    let src = "  \n    \r   \t  ";
+    assert_eq!(None, SourceIter(src).next());
+
+    let src = "7";
+    assert_eq!(Num(7), SourceIter(src).next().unwrap().0.value);
+
+    let src = "     100         ";
+    assert_eq!(Num(100), SourceIter(src).next().unwrap().0.value);
+
+    let src = "     9a         ";
+    assert_eq!(Err(Unexpected('a')), SourceIter(src).next().unwrap().0.value);
+
+    let src = "     :123foo      ";
+    assert_eq!(Sym(":123foo"), SourceIter(src).next().unwrap().0.value);
+
+    let src = " \r\r\r\n\n\n@bar456\t\t\t\t\t\t";
+    assert_eq!(Sym("@bar456"), SourceIter(src).next().unwrap().0.value);
+
+    let src = "foo123";
+    assert_eq!(Key("foo123"), SourceIter(src).next().unwrap().0.value);
+
+    let src = "foo/bar";
+    assert_eq!(Key("foo/bar"), SourceIter(src).next().unwrap().0.value);
+
+    let src = "\"foo/bar\"";
+    assert_eq!(Str("foo/bar"), SourceIter(src).next().unwrap().0.value);
+
+    let src = "    \"foo/bar\"    ";
+    assert_eq!(Str("foo/bar"), SourceIter(src).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(&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
new file mode 100644
index 0000000..4411e7e
--- /dev/null
+++ b/dsl/src/dsl_token.rs
@@ -0,0 +1,25 @@
+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
new file mode 100644
index 0000000..45b586f
--- /dev/null
+++ b/dsl/src/dsl_value.rs
@@ -0,0 +1,84 @@
+use crate::*;
+use std::fmt::Display;
+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 0cd66c8..7e990c8 100644
--- a/dsl/src/lib.rs
+++ b/dsl/src/lib.rs
@@ -1,7 +1,3 @@
-#![feature(adt_const_params)]
-#![feature(type_alias_impl_trait)]
-#![feature(impl_trait_in_fn_trait_return)]
-
 //! [Token]s are parsed substrings with an associated [Value].
 //!
 //! * [ ] FIXME: Value may be [Err] which may shadow [Result::Err]
@@ -35,141 +31,21 @@
 //! assert_eq!(view.0.0, src);
 //! assert_eq!(view.peek(), view.0.peek())
 //! ```
-
+#![feature(adt_const_params)]
+#![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 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_domain;  pub use self::dsl_domain::*;
 mod dsl_error;   pub use self::dsl_error::*;
-
-#[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();
-        let mut iter = crate::TokenIter::new(&":foo :bar");
-        let _ = iter.next();
-    }
-    #[test] const fn test_const_iters () {
-        let mut iter = crate::SourceIter::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 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 = crate::Token {
-                source: &source,
-                start,
-                length,
-                value: crate::Value::Nil
-            };
-            let _ = token.slice();
-        }
-    }
-}
-
-#[cfg(test)] #[test] fn test_token () -> Result<(), Box<dyn std::error::Error>> {
-    let source = ":f00";
-    let mut token = Token { source, start: 0, length: 1, value: Sym(":") };
-    token = token.grow_sym();
-    assert_eq!(token, Token { source, start: 0, length: 2, value: Sym(":f") }); 
-    token = token.grow_sym();
-    assert_eq!(token, Token { source, start: 0, length: 3, value: Sym(":f0") }); 
-    token = token.grow_sym();
-    assert_eq!(token, Token { source, start: 0, length: 4, value: Sym(":f00") }); 
-
-    let src = "";
-    assert_eq!(None, SourceIter(src).next());
-
-    let src = "  \n    \r   \t  ";
-    assert_eq!(None, SourceIter(src).next());
-
-    let src = "7";
-    assert_eq!(Num(7), SourceIter(src).next().unwrap().0.value);
-
-    let src = "     100         ";
-    assert_eq!(Num(100), SourceIter(src).next().unwrap().0.value);
-
-    let src = "     9a         ";
-    assert_eq!(Err(Unexpected('a')), SourceIter(src).next().unwrap().0.value);
-
-    let src = "     :123foo      ";
-    assert_eq!(Sym(":123foo"), SourceIter(src).next().unwrap().0.value);
-
-    let src = " \r\r\r\n\n\n@bar456\t\t\t\t\t\t";
-    assert_eq!(Sym("@bar456"), SourceIter(src).next().unwrap().0.value);
-
-    let src = "foo123";
-    assert_eq!(Key("foo123"), SourceIter(src).next().unwrap().0.value);
-
-    let src = "foo/bar";
-    assert_eq!(Key("foo/bar"), SourceIter(src).next().unwrap().0.value);
-
-    let src = "\"foo/bar\"";
-    assert_eq!(Str("foo/bar"), SourceIter(src).next().unwrap().0.value);
-
-    let src = "    \"foo/bar\"    ";
-    assert_eq!(Str("foo/bar"), SourceIter(src).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(&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(())
-//}
+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;
diff --git a/input/src/input_dsl.rs b/input/src/input_dsl.rs
index c6d26e1..a91453f 100644
--- a/input/src/input_dsl.rs
+++ b/input/src/input_dsl.rs
@@ -1,14 +1,12 @@
 use crate::*;
 use std::marker::PhantomData;
 use std::fmt::Debug;
-/// List of input layers with optional conditional filters.
-#[derive(Default, Debug)] pub struct InputLayers<S>(Vec<InputLayer<S>>);
-#[derive(Default, Debug)] pub struct InputLayer<S>{
-    __:        PhantomData<S>,
-    condition: Option<Ast>,
-    binding:   Ast,
-}
-impl<S> InputLayers<S> {
+
+#[derive(Default, Debug)] pub struct InputLayers(Vec<InputLayer>);
+
+#[derive(Default, Debug)] pub struct InputLayer(Option<Ast>, Ast);
+
+impl InputLayers {
     pub fn new (layer: Ast) -> Self {
         Self(vec![]).layer(layer)
     }
@@ -22,30 +20,30 @@ impl<S> InputLayers<S> {
         self.add_layer_if(None, layer.into()); self
     }
     pub fn add_layer_if (&mut self, condition: Option<Ast>, binding: Ast) -> &mut Self {
-        self.0.push(InputLayer { condition, binding, __: Default::default() });
+        self.0.push(InputLayer(condition, binding));
         self
     }
+    pub fn handle <S: Eval<Ast, bool> + Eval<Ast, O>, I: Eval<Ast, bool>, O: Command<S>> (&self, state: &mut S, input: I) -> Perhaps<O> {
+        InputHandle(state, self.0.as_slice()).try_eval(input)
+    }
 }
-impl<S: Eval<Ast, bool> + Eval<Ast, C>, C: Command<S>, I: Debug + Eval<Ast, bool>> Eval<(S, I), C> for InputLayers<S> {
-    fn try_eval (&self, (state, input): (S, I)) -> Perhaps<C> {
-        for InputLayer { condition, binding, .. } in self.0.iter() {
+
+pub struct InputHandle<'a, S>(&'a mut S, &'a [InputLayer]);
+
+impl<'a, S: Eval<Ast, bool> + Eval<Ast, O>, I: Eval<Ast, bool>, O: Command<S>> Eval<I, O> for InputHandle<'a, S> {
+    fn try_eval (&self, input: I) -> Perhaps<O> {
+        let Self(state, layers) = self;
+        for InputLayer(condition, binding) in layers.iter() {
             let mut matches = true;
             if let Some(condition) = condition {
                 matches = state.eval(condition.clone(), ||"input: no condition")?;
             }
-            if matches {
-                if let Ast::Exp(e) = binding {
-                    if let Some(ast) = e.peek() {
-                        if input.eval(ast.clone(), ||"InputLayers: input.eval(binding) failed")?
-                        && let Some(command) = state.try_eval(ast)? {
-                            return Ok(Some(command))
-                        }
-                    } else {
-                        unreachable!("InputLayer")
-                    }
-                } else {
-                    panic!("InputLayer: expected expression, got: {input:?}")
-                }
+            if matches
+            && let Some(exp) = binding.exp()
+            && let Some(ast) = exp.peek()
+            && input.eval(ast.clone(), ||"InputLayers: input.eval(binding) failed")?
+            && let Some(command) = state.try_eval(ast)? {
+                return Ok(Some(command))
             }
         }
         Ok(None)
diff --git a/input/src/lib.rs b/input/src/lib.rs
index 21035fe..4f27011 100644
--- a/input/src/lib.rs
+++ b/input/src/lib.rs
@@ -1,4 +1,5 @@
 #![feature(associated_type_defaults)]
+#![feature(if_let_guard)]
 
 pub(crate) use tengri_core::*;
 
diff --git a/output/src/ops_dsl.rs b/output/src/ops_dsl.rs
index e86a634..196f693 100644
--- a/output/src/ops_dsl.rs
+++ b/output/src/ops_dsl.rs
@@ -1,159 +1,118 @@
 use crate::*;
+use Value::*;
 
 impl<S, A> Dsl<S> for When<A> where S: Eval<Ast, bool> + Eval<Ast, A> {
-    fn try_provide (state: S, source: Ast) -> Perhaps<Self> {
-        if let Ast::Exp(source) = source {
-            if let Some(Ast::Key(id)) = source.next() && *id == *"when" {
-                return Ok(Some(Self(
-                    state.eval(source.next().unwrap(), ||"when: expected condition")?,
-                    state.eval(source.next().unwrap(), ||"when: expected content")?,
-                )))
-            }
+    fn try_provide (state: &S, source: Ast) -> Perhaps<Self> {
+        if let Exp(_, exp) = source.0 && 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")?,
+            )))
         }
         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, mut source: Ast) -> Perhaps<Self> {
-        if let Ast::Exp(mut source) = source {
-            if let Some(Ast::Key(id)) = source.next() && *id == *"either" {
-                return Ok(Some(Self(
-                    state.eval(source.next().unwrap(), ||"either: expected condition")?,
-                    state.eval(source.next().unwrap(), ||"either: expected content 1")?,
-                    state.eval(source.next().unwrap(), ||"either: expected content 2")?,
-                )))
-            }
+    fn try_provide (state: &S, source: Ast) -> Perhaps<Self> {
+        if let Exp(_, exp) = source.0 && 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, B> Dsl<S> for Bsp<A, B>
-where S: Eval<Option<Ast>, A> + Eval<Option<Ast>, B> {
-    fn try_provide (state: S, source: Ast) -> Perhaps<Self> {
-        Ok(Some(if let Ast::Exp(source) = source {
-            match source.next() {
-                Some(Value::Key("bsp/n")) => {
-                    let a: A = state.eval(source.next(), ||"bsp/n: expected content 1")?;
-                    let b: B = state.eval(source.next(), ||"bsp/n: expected content 2")?;
-                    Self::n(a, b)
-                },
-                Some(Value::Key("bsp/s")) => {
-                    let a: A = state.eval(source.next(), ||"bsp/s: expected content 1")?;
-                    let b: B = state.eval(source.next(), ||"bsp/s: expected content 2")?;
-                    Self::s(a, b)
-                },
-                Some(Value::Key("bsp/e")) => {
-                    let a: A = state.eval(source.next(), ||"bsp/e: expected content 1")?;
-                    let b: B = state.eval(source.next(), ||"bsp/e: expected content 2")?;
-                    Self::e(a, b)
-                },
-                Some(Value::Key("bsp/w")) => {
-                    let a: A = state.eval(source.next(), ||"bsp/w: expected content 1")?;
-                    let b: B = state.eval(source.next(), ||"bsp/w: expected content 2")?;
-                    Self::w(a, b)
-                },
-                Some(Value::Key("bsp/a")) => {
-                    let a: A = state.eval(source.next(), ||"bsp/a: expected content 1")?;
-                    let b: B = state.eval(source.next(), ||"bsp/a: expected content 2")?;
-                    Self::a(a, b)
-                },
-                Some(Value::Key("bsp/b")) => {
-                    let a: A = state.eval(source.next(), ||"bsp/b: expected content 1")?;
-                    let b: B = state.eval(source.next(), ||"bsp/b: expected content 2")?;
-                    Self::b(a, b)
-                },
+impl<S, A> Dsl<S> for Align<A> where S: Eval<Option<Ast>, A> {
+    fn try_provide (state: &S, source: Ast) -> Perhaps<Self> {
+        if let Exp(_, source) = source.0 {
+            let mut rest = source.clone();
+            return Ok(Some(match rest.next().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),
-            }
-        } else {
-            return Ok(None)
-        }))
+            }))
+        }
+        Ok(None)
     }
 }
 
-impl<S, A> Dsl<S> for Align<A> where S: Eval<Option<Ast>, A> {
-    fn try_provide (state: S, source: Ast) -> Perhaps<Self> {
-        Ok(Some(if let Ast::Exp(source) = source {
-            match source.next() {
-                Some(Value::Key("align/c"))  => {
-                    let content: A = state.eval(source.next(), ||"align/c: expected content")?;
-                    Self::c(content)
-                },
-                Some(Value::Key("align/x"))  => {
-                    let content: A = state.eval(source.next(), ||"align/x: expected content")?;
-                    Self::x(content)
-                },
-                Some(Value::Key("align/y"))  => {
-                    let content: A = state.eval(source.next(), ||"align/y: expected content")?;
-                    Self::y(content)
-                },
-                Some(Value::Key("align/n"))  => {
-                    let content: A = state.eval(source.next(), ||"align/n: expected content")?;
-                    Self::n(content)
-                },
-                Some(Value::Key("align/s"))  => {
-                    let content: A = state.eval(source.next(), ||"align/s: expected content")?;
-                    Self::s(content)
-                },
-                Some(Value::Key("align/e"))  => {
-                    let content: A = state.eval(source.next(), ||"align/e: expected content")?;
-                    Self::e(content)
-                },
-                Some(Value::Key("align/w"))  => {
-                    let content: A = state.eval(source.next(), ||"align/w: expected content")?;
-                    Self::w(content)
-                },
-                Some(Value::Key("align/nw")) => {
-                    let content: A = state.eval(source.next(), ||"align/nw: expected content")?;
-                    Self::nw(content)
-                },
-                Some(Value::Key("align/ne")) => {
-                    let content: A = state.eval(source.next(), ||"align/ne: expected content")?;
-                    Self::ne(content)
-                },
-                Some(Value::Key("align/sw")) => {
-                    let content: A = state.eval(source.next(), ||"align/sw: expected content")?;
-                    Self::sw(content)
-                },
-                Some(Value::Key("align/se")) => {
-                    let content: A = state.eval(source.next(), ||"align/se: expected content")?;
-                    Self::se(content)
-                },
+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: Ast) -> Perhaps<Self> {
+        if let Exp(_, exp) = source.0 {
+            let mut rest = exp.clone();
+            return Ok(Some(match rest.next().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),
-            }
-        } else {
-            return Ok(None)
-        }))
+            }))
+        }
+        Ok(None)
     }
 }
 
         //#[cfg(feature = "dsl")] take!($Enum<A>, A|state, words|Ok(
-            //if let Some(Token { value: Value::Key(k), .. }) = words.peek() {
+            //if let Some(Token { value: Key(k), .. }) = words.peek() {
                 //let mut base = words.clone();
                 //let content = state.give_or_fail(words, ||format!("{k}: no content"))?;
                 //return Ok(Some(match words.next() {
-                    //Some(Token{value: Value::Key($x),..})  => Self::x(content),
-                    //Some(Token{value: Value::Key($y),..})  => Self::y(content),
-                    //Some(Token{value: Value::Key($xy),..}) => Self::xy(content),
+                    //Some(Token{value: Key($x),..})  => Self::x(content),
+                    //Some(Token{value: Key($y),..})  => Self::y(content),
+                    //Some(Token{value: Key($xy),..}) => Self::xy(content),
                     //_ => unreachable!()
                 //}))
             //} else {
                 //None
             //}));
         //#[cfg(feature = "dsl")] take!($Enum<U, A>, U, A|state, words|Ok(
-            //if let Some(Token { value: Value::Key($x|$y|$xy), .. }) = words.peek() {
+            //if let Some(Token { value: Key($x|$y|$xy), .. }) = words.peek() {
                 //let mut base = words.clone();
                 //Some(match words.next() {
-                    //Some(Token { value: Value::Key($x), .. }) => Self::x(
+                    //Some(Token { value: Key($x), .. }) => Self::x(
                         //state.give_or_fail(words, ||"x: no unit")?,
                         //state.give_or_fail(words, ||"x: no content")?,
                     //),
-                    //Some(Token { value: Value::Key($y), .. }) => Self::y(
+                    //Some(Token { value: Key($y), .. }) => Self::y(
                         //state.give_or_fail(words, ||"y: no unit")?,
                         //state.give_or_fail(words, ||"y: no content")?,
                     //),
-                    //Some(Token { value: Value::Key($x), .. }) => Self::xy(
+                    //Some(Token { value: Key($x), .. }) => Self::xy(
                         //state.give_or_fail(words, ||"xy: no unit x")?,
                         //state.give_or_fail(words, ||"xy: no unit y")?,
                         //state.give_or_fail(words, ||"xy: no content")?