diff --git a/output/src/ops.rs b/output/src/ops.rs
index 7eba655..7b8010e 100644
--- a/output/src/ops.rs
+++ b/output/src/ops.rs
@@ -1,9 +1,10 @@
+//mod reduce;    pub use self::reduce::*;
 mod align;     pub use self::align::*;
 mod bsp;       pub use self::bsp::*;
-mod cond;      pub use self::cond::*;
+mod either;    pub use self::either::*;
 mod map;       pub use self::map::*;
 mod memo;      pub use self::memo::*;
 mod stack;     pub use self::stack::*;
-//mod reduce;    pub use self::reduce::*;
 mod thunk;     pub use self::thunk::*;
 mod transform; pub use self::transform::*;
+mod when;      pub use self::when::*;
diff --git a/output/src/ops/either.rs b/output/src/ops/either.rs
new file mode 100644
index 0000000..75c6ccf
--- /dev/null
+++ b/output/src/ops/either.rs
@@ -0,0 +1,38 @@
+use crate::*;
+
+/// Show one item if a condition is true and another if the condition is false
+pub struct Either<A, B>(pub bool, pub A, pub B);
+impl<A, B> Either<A, B> {
+    /// Create a ternary condition.
+    pub const fn new (c: bool, a: A, b: B) -> Self {
+        Self(c, a, b)
+    }
+}
+#[cfg(feature = "dsl")]
+impl<A, B, T: Dsl<bool> + Dsl<A> + Dsl<B>> Namespace<T> for Either<A, B> {
+    fn take_from <'source> (
+        state: &T,
+        token: &mut TokenIter<'source>
+    ) -> Perhaps<Self> {
+        if let Some(Token { value: Value::Key("either"), .. }) = token.peek() {
+            let base = token.clone();
+            let _ = token.next().unwrap();
+            return Ok(Some(Self(
+                state.take_or_fail(token, "either: no condition")?,
+                state.take_or_fail(token, "either: no content 1")?,
+                state.take_or_fail(token, "either: no content 2")?,
+            )))
+        }
+        Ok(None)
+    }
+}
+impl<E: Output, A: Render<E>, B: Render<E>> Content<E> for Either<A, B> {
+    fn layout (&self, to: E::Area) -> E::Area {
+        let Self(cond, a, b) = self;
+        if *cond { a.layout(to) } else { b.layout(to) }
+    }
+    fn render (&self, to: &mut E) {
+        let Self(cond, a, b) = self;
+        if *cond { a.render(to) } else { b.render(to) }
+    }
+}
diff --git a/output/src/ops/cond.rs b/output/src/ops/when.rs
similarity index 51%
rename from output/src/ops/cond.rs
rename to output/src/ops/when.rs
index be950fe..188a5b5 100644
--- a/output/src/ops/cond.rs
+++ b/output/src/ops/when.rs
@@ -8,15 +8,6 @@ impl<A> When<A> {
         Self(c, a)
     }
 }
-
-/// Show one item if a condition is true and another if the condition is false
-pub struct Either<A, B>(pub bool, pub A, pub B);
-impl<A, B> Either<A, B> {
-    /// Create a ternary condition.
-    pub const fn new (c: bool, a: A, b: B) -> Self {
-        Self(c, a, b)
-    }
-}
 #[cfg(feature = "dsl")]
 impl<A, T: Dsl<bool> + Dsl<A>> Namespace<T> for When<A> {
     fn take_from <'source> (
@@ -55,31 +46,3 @@ impl<E: Output, A: Render<E>> Content<E> for When<A> {
         if *cond { item.render(to) }
     }
 }
-#[cfg(feature = "dsl")]
-impl<A, B, T: Dsl<bool> + Dsl<A> + Dsl<B>> Namespace<T> for Either<A, B> {
-    fn take_from <'source> (
-        state: &T,
-        token: &mut TokenIter<'source>
-    ) -> Perhaps<Self> {
-        if let Some(Token { value: Value::Key("either"), .. }) = token.peek() {
-            let base = token.clone();
-            let _ = token.next().unwrap();
-            return Ok(Some(Self(
-                state.take_or_fail(token, "either: no condition")?,
-                state.take_or_fail(token, "either: no content 1")?,
-                state.take_or_fail(token, "either: no content 2")?,
-            )))
-        }
-        Ok(None)
-    }
-}
-impl<E: Output, A: Render<E>, B: Render<E>> Content<E> for Either<A, B> {
-    fn layout (&self, to: E::Area) -> E::Area {
-        let Self(cond, a, b) = self;
-        if *cond { a.layout(to) } else { b.layout(to) }
-    }
-    fn render (&self, to: &mut E) {
-        let Self(cond, a, b) = self;
-        if *cond { a.render(to) } else { b.render(to) }
-    }
-}
diff --git a/proc/src/proc_view.rs b/proc/src/proc_view.rs
index 7223dc0..ac18a80 100644
--- a/proc/src/proc_view.rs
+++ b/proc/src/proc_view.rs
@@ -42,50 +42,53 @@ impl Parse for ViewImpl {
 impl ToTokens for ViewDef {
     fn to_tokens (&self, out: &mut TokenStream2) {
         let Self(ViewMeta { output }, ViewImpl { block, exposed }) = self;
-        let view = &block.self_ty;
-        let builtins = builtins().iter().map(|ty|write_quote(quote! {
-            if let Some(value) = Namespace::<#ty>::take_from(state, &mut exp.clone())? {
-                return Ok(Some(value.boxed()))
-            }
-        })).collect::<Vec<_>>();
-        let mut available = vec![];
-        let exposed: Vec<_> = exposed.iter().map(|(key, value)|{
-            available.push(key.clone());
-            write_quote(quote! { #key => Some(#view::#value(state).boxed()), })
-        }).collect();
-        let available: String = available.join(", ");
-        let error_msg = LitStr::new(
-            &format!("expected Sym(content), got: {{token:?}}, available: {available}"),
-            Span::call_site()
-        );
+        let self_ty = &block.self_ty;
+        let builtins: Vec<_> = builtins_with_types()
+            .iter()
+            .map(|ty|write_quote(quote! {
+                let value: Option<#ty> = Dsl::take(state, &mut exp.clone())?;
+                if let Some(value) = value {
+                    return Ok(Some(value.boxed()))
+                }
+            }))
+        .collect();
+        let exposed: Vec<_> = exposed
+            .iter()
+            .map(|(key, value)|write_quote(quote! {
+                #key => Some(#self_ty::#value(state).boxed()),
+            })).collect();
         write_quote_to(out, quote! {
             #block
             /// Generated by [tengri_proc].
             ///
-            /// Delegates the rendering of [#view] to the [#view::view} method,
+            /// Delegates the rendering of [#self_ty] to the [#self_ty::view} method,
             /// which you will need to implement, e.g. passing a [TokenIter]
             /// containing a layout and keybindings config from user dirs.
-            impl ::tengri::output::Content<#output> for #view {
+            impl ::tengri::output::Content<#output> for #self_ty {
                 fn content (&self) -> impl Render<#output> {
-                    self.view()
+                    #self_ty::view(self)
                 }
             }
             /// Generated by [tengri_proc].
             ///
-            /// Gives [#view] the ability to construct the [Render]able
+            /// Gives [#self_ty] the ability to construct the [Render]able
             /// which might corresponds to a given [TokenStream],
-            /// while taking [#view]'s state into consideration.
-            impl ::tengri::dsl::Namespace<#view> for Box<dyn Render<#output>> {
+            /// while taking [#self_ty]'s state into consideration.
+            impl ::tengri::dsl::Namespace<#self_ty> for Box<dyn Render<#output> + '_> {
                 fn take_from <'source> (
-                    state: &#view,
+                    state: &#self_ty,
                     words: &mut ::tengri::dsl::TokenIter<'source>
                 ) -> Perhaps<Self> {
                     Ok(if let Some(::tengri::dsl::Token { value, .. }) = words.peek() {
                         match value {
+                            // Expressions are handled by built-in functions
+                            // that operate over constants and symbols.
                             ::tengri::dsl::Value::Exp(_, exp) => {
-                                //#(#builtins)*
+                                #(#builtins)*
                                 None
                             },
+                            // Symbols are handled by user-provided functions
+                            // that take no parameters but `&self`.
                             ::tengri::dsl::Value::Sym(sym) => match sym {
                                 #(#exposed)*
                                 _ => None
@@ -101,21 +104,21 @@ impl ToTokens for ViewDef {
     }
 }
 
-fn builtins () -> [TokenStream2;14] {
+fn builtins_with_types () -> [TokenStream2;14] {
     [
-        quote! { When::<_>       },
-        quote! { Either::<_, _>  },
-        quote! { Align::<_>      },
-        quote! { Bsp::<_, _>     },
-        quote! { Fill::<_>       },
-        quote! { Fixed::<_, _>   },
-        quote! { Min::<_, _>     },
-        quote! { Max::<_, _>     },
-        quote! { Shrink::<_, _>  },
-        quote! { Expand::<_, _>  },
-        quote! { Push::<_, _>    },
-        quote! { Pull::<_, _>    },
-        quote! { Margin::<_, _>  },
-        quote! { Padding::<_, _> },
+        quote! { When<      Box<dyn Render<_> + '_>                         > },
+        quote! { Either<    Box<dyn Render<_> + '_>, Box<dyn Render<_> + '_>> },
+        quote! { Align<     Box<dyn Render<_> + '_>                         > },
+        quote! { Bsp<       Box<dyn Render<_> + '_>, Box<dyn Render<_> + '_>> },
+        quote! { Fill<      Box<dyn Render<_> + '_>                         > },
+        quote! { Fixed<_,   Box<dyn Render<_> + '_>                         > },
+        quote! { Min<_,     Box<dyn Render<_> + '_>                         > },
+        quote! { Max<_,     Box<dyn Render<_> + '_>                         > },
+        quote! { Shrink<_,  Box<dyn Render<_> + '_>                         > },
+        quote! { Expand<_,  Box<dyn Render<_> + '_>                         > },
+        quote! { Push<_,    Box<dyn Render<_> + '_>                         > },
+        quote! { Pull<_,    Box<dyn Render<_> + '_>                         > },
+        quote! { Margin<_,  Box<dyn Render<_> + '_>                         > },
+        quote! { Padding<_, Box<dyn Render<_> + '_>                         > },
     ]
 }