separate Input and Output impls

engine/src/thunk.rs

@@ -0,0 +1,10 @@
+use crate::*;
+use std::marker::PhantomData;
+/// Lazily-evaluated [Render]able.
+pub struct Thunk<E: Output, T: Render<E>, F: Fn()->T + Send + Sync>(F, PhantomData<E>);
+impl<E: Output, T: Render<E>, F: Fn()->T + Send + Sync> Thunk<E, T, F> {
+    pub fn new (thunk: F) -> Self { Self(thunk, Default::default()) }
+}
+impl<E: Output, T: Render<E>, F: Fn()->T + Send + Sync> Content<E> for Thunk<E, T, F> {
+    fn content (&self) -> impl Render<E> { (self.0)() }
+}