diff --git a/prototype/Doubt/Patch.swift b/prototype/Doubt/Patch.swift
index 6a195364d..8034c65e2 100644
--- a/prototype/Doubt/Patch.swift
+++ b/prototype/Doubt/Patch.swift
@@ -64,6 +64,7 @@ extension Patch {
 }
 
 
+/// A hack to enable constrained extensions on `Free<A, Patch<A>>`.
 public protocol PatchConvertible {
 	typealias Info