diff --git a/dsl/src/dsl_ast.rs b/dsl/src/dsl_ast.rs
index 866e15f..fca3fd6 100644
--- a/dsl/src/dsl_ast.rs
+++ b/dsl/src/dsl_ast.rs
@@ -1,16 +1,8 @@
 use crate::*;
 use std::sync::Arc;
 use std::borrow::Cow;
-
-#[derive(Debug, Copy, Clone, Default, PartialEq)]
-pub struct AstIter;
-impl<'source> From<SourceIter<'source>> for AstIter {
-    fn from (source: SourceIter<'source>) -> Self {
-        Self // TODO
-    }
-}
-/// Owns its values, and has no metadata.
-pub type Ast = Value<Arc<str>, AstIter>;
+#[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()
@@ -19,7 +11,7 @@ impl<'source> From<CstToken<'source>> for Ast {
 impl<'source> From<CstValue<'source>> for Ast {
     fn from (value: CstValue<'source>) -> Self {
         use Value::*;
-        match value {
+        Self(match value {
             Nil    => Nil,
             Err(e) => Err(e),
             Num(u) => Num(u),
@@ -27,6 +19,14 @@ impl<'source> From<CstValue<'source>> for Ast {
             Key(s) => Key(s.into()),
             Str(s) => Str(s.into()),
             Exp(d, x) => Exp(d, x.into()),
-        }
+        })
+    }
+}
+
+#[derive(Debug, Copy, Clone, Default, PartialEq)]
+pub struct AstIter; // TODO
+impl<'source> From<SourceIter<'source>> for AstIter {
+    fn from (source: SourceIter<'source>) -> Self {
+        Self // TODO
     }
 }
diff --git a/dsl/src/dsl_cst.rs b/dsl/src/dsl_cst.rs
index 1c896b5..1af6f8f 100644
--- a/dsl/src/dsl_cst.rs
+++ b/dsl/src/dsl_cst.rs
@@ -1,13 +1,12 @@
 use crate::*;
-
-/// Keeps the reference to the source slice.
+/// 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,
 }
-
-pub type CstValue<'source> = Value<&'source str, SourceIter<'source>>;
diff --git a/dsl/src/dsl_display.rs b/dsl/src/dsl_display.rs
index 44def5d..aaae8c7 100644
--- a/dsl/src/dsl_display.rs
+++ b/dsl/src/dsl_display.rs
@@ -3,7 +3,7 @@ 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 {
+        write!(out, "{}", match &self.0 {
             Nil => String::new(),
             Err(e) => format!("[error: {e}]"),
             Num(n) => format!("{n}"),
diff --git a/input/src/input_dsl.rs b/input/src/input_dsl.rs
index 6f6b24b..bff457d 100644
--- a/input/src/input_dsl.rs
+++ b/input/src/input_dsl.rs
@@ -23,14 +23,14 @@ impl InputLayers {
         self.0.push(InputLayer(condition, binding));
         self
     }
-    pub fn handle <S, I, O: Command<S>> (&self, state: &mut S, input: I) -> Perhaps<O> {
+    pub fn handle <S: Eval<Ast, bool>, I, O: Command<S>> (&self, state: &mut S, input: I) -> Perhaps<O> {
         InputHandle(state, self.0.as_slice()).try_eval(input)
     }
 }
 
 pub struct InputHandle<'a, S>(&'a mut S, &'a [InputLayer]);
 
-impl<'a, S, I, O: Command<S>> Eval<I, O> for InputHandle<'a, S> {
+impl<'a, S: Eval<Ast, bool>, I, 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() {