diff --git a/src/func/instances/composition.rs b/src/func/instances/composition.rs
index e19384a..fdf9a66 100644
--- a/src/func/instances/composition.rs
+++ b/src/func/instances/composition.rs
@@ -1,3 +1,5 @@
+//! [Functor]s derived on compositions of other [Functor]s.
+
 use crate::func::*;
 
 pub struct CompositionInstance<U, V>(U, V);
@@ -26,6 +28,7 @@ impl<'a, U: Pure<'a>, V: Pure<'a>> Pure<'a> for CompositionInstance<U, V> {
     }
 }
 
+/// Note: depends on the outer instance being [`ApplicativeLA2`] not [`ApplicativeSeq`].
 impl<'a, U: ApplicativeLA2<'a>, V: ApplicativeSeq<'a>> ApplicativeSeq<'a>
     for CompositionInstance<U, V>
 {
@@ -54,6 +57,7 @@ impl<'a, U: ApplicativeTuple<'a>, V: ApplicativeTuple<'a>> ApplicativeTuple<'a>
     }
 }
 
+/// Note: depends on the inner instance being [`Functor`] not [`ApplicativeSelect`].
 impl<'a, U: ApplicativeSelect<'a>, V: Functor<'a>> ApplicativeSelect<'a>
     for CompositionInstance<U, V>
 {
@@ -87,6 +91,13 @@ impl<'a, U: Monad<'a>, V: Monad<'a> + LocalFunctor<'a>> Monad<'a> for Compositio
         U::iterate(ComposedIterative(f))
     }
 
+    /// Basis for how composition can be a [Monad].
+    ///
+    /// ```text
+    /// U V U V A
+    /// U U V V A
+    ///  U   V  A
+    /// ```
     fn join<A: 'a>(ffa: Self::F<Self::F<A>>) -> Self::F<A> {
         U::join(U::fmap(|ufa| U::fmap(V::join, V::stuff::<_, U>(ufa)), ffa))
     }
@@ -116,6 +127,7 @@ impl<
     }
 }
 
+/// Note: fails in the inner instance not outer.
 impl<'a, E: 'a, U: Monad<'a>, V: Fail<'a, E> + LocalFunctor<'a>> Fail<'a, E>
     for CompositionInstance<U, V>
 {