diff --git a/prototype/Doubt/Equatable.swift b/prototype/Doubt/Equatable.swift
index 34cfd49e7..e134da62d 100644
--- a/prototype/Doubt/Equatable.swift
+++ b/prototype/Doubt/Equatable.swift
@@ -60,3 +60,14 @@ public func == (left: Doc, right: Doc) -> Bool {
 		return false
 	}
 }
+
+public func == <A: Equatable> (left: Vertex<A>, right: Vertex<A>) -> Bool {
+	switch (left, right) {
+	case (.End, .End):
+		return true
+	case let (.XY(a, x1, y1), .XY(b, x2, y2)):
+		return a == b && x1.value == y1.value && x2.value == y2.value
+	default:
+		return false
+	}
+}