diff --git a/prototype/Doubt/Algorithm.swift b/prototype/Doubt/Algorithm.swift
index 228d3ac4f..407b49360 100644
--- a/prototype/Doubt/Algorithm.swift
+++ b/prototype/Doubt/Algorithm.swift
@@ -1,6 +1,9 @@
 /// A language of diffing algorithms.
 enum Algorithm<Recur, A> {
+	/// The type of `Term`s over which `Algorithm`s operate.
 	typealias Term = Fix<A>
+
+	/// The type of `Diff`s which `Algorithm`s produce.
 	typealias Diff = Free<A, Patch<A>>
 
 	case Recursive(Term, Diff -> Recur)