diff --git a/prototype/Doubt.xcodeproj/project.pbxproj b/prototype/Doubt.xcodeproj/project.pbxproj
index 488954213..8b23866e5 100644
--- a/prototype/Doubt.xcodeproj/project.pbxproj
+++ b/prototype/Doubt.xcodeproj/project.pbxproj
@@ -22,6 +22,7 @@
 		D45A36C91BBC667D00BE3DDE /* Category.swift in Sources */ = {isa = PBXBuildFile; fileRef = D45A36C81BBC667D00BE3DDE /* Category.swift */; settings = {ASSET_TAGS = (); }; };
 		D45A36CD1BBC75DF00BE3DDE /* Info.swift in Sources */ = {isa = PBXBuildFile; fileRef = D45A36CC1BBC75DF00BE3DDE /* Info.swift */; settings = {ASSET_TAGS = (); }; };
 		D49C2CA41BBD72E300949127 /* AnyHashable.swift in Sources */ = {isa = PBXBuildFile; fileRef = D49C2CA31BBD72E300949127 /* AnyHashable.swift */; settings = {ASSET_TAGS = (); }; };
+		D49FCBAD1BBD7A3100C5E9C3 /* AnyEquatable.swift in Sources */ = {isa = PBXBuildFile; fileRef = D49FCBAC1BBD7A3100C5E9C3 /* AnyEquatable.swift */; settings = {ASSET_TAGS = (); }; };
 		D4A71DC51BB45B850051416D /* Vertex.swift in Sources */ = {isa = PBXBuildFile; fileRef = D4A71DC41BB45B850051416D /* Vertex.swift */; settings = {ASSET_TAGS = (); }; };
 		D4A71DC71BB4AC9E0051416D /* VertexTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = D4A71DC61BB4AC9E0051416D /* VertexTests.swift */; settings = {ASSET_TAGS = (); }; };
 		D4AAE50E1B5AE22E004E581F /* Doubt.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = D4AAE4FD1B5AE22E004E581F /* Doubt.framework */; };
@@ -100,6 +101,7 @@
 		D45A36C81BBC667D00BE3DDE /* Category.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Category.swift; sourceTree = "<group>"; };
 		D45A36CC1BBC75DF00BE3DDE /* Info.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Info.swift; sourceTree = "<group>"; };
 		D49C2CA31BBD72E300949127 /* AnyHashable.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = AnyHashable.swift; sourceTree = "<group>"; };
+		D49FCBAC1BBD7A3100C5E9C3 /* AnyEquatable.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = AnyEquatable.swift; sourceTree = "<group>"; };
 		D4A71DC41BB45B850051416D /* Vertex.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Vertex.swift; sourceTree = "<group>"; };
 		D4A71DC61BB4AC9E0051416D /* VertexTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = VertexTests.swift; sourceTree = "<group>"; };
 		D4AAE4FD1B5AE22E004E581F /* Doubt.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = Doubt.framework; sourceTree = BUILT_PRODUCTS_DIR; };
@@ -208,6 +210,7 @@
 				D45A36C81BBC667D00BE3DDE /* Category.swift */,
 				D45A36CC1BBC75DF00BE3DDE /* Info.swift */,
 				D49C2CA31BBD72E300949127 /* AnyHashable.swift */,
+				D49FCBAC1BBD7A3100C5E9C3 /* AnyEquatable.swift */,
 				D4AAE5001B5AE22E004E581F /* Supporting Files */,
 			);
 			path = Doubt;
@@ -396,6 +399,7 @@
 				D432D4731BA9C55300F3FABC /* Stream.swift in Sources */,
 				D4AAE5421B5AE2D0004E581F /* Doc.swift in Sources */,
 				D49C2CA41BBD72E300949127 /* AnyHashable.swift in Sources */,
+				D49FCBAD1BBD7A3100C5E9C3 /* AnyEquatable.swift in Sources */,
 				D432D4771BA9FE6A00F3FABC /* Comparable.swift in Sources */,
 				D45A36CD1BBC75DF00BE3DDE /* Info.swift in Sources */,
 				D4AAE5461B5AE2D0004E581F /* Parse.swift in Sources */,
diff --git a/prototype/Doubt/AnyEquatable.swift b/prototype/Doubt/AnyEquatable.swift
new file mode 100644
index 000000000..0be914841
--- /dev/null
+++ b/prototype/Doubt/AnyEquatable.swift
@@ -0,0 +1,10 @@
+public enum AnyEquatable<A>: Equatable {
+	case External(A, (A, A) -> Bool)
+}
+
+public func == <A> (left: AnyEquatable<A>, right: AnyEquatable<A>) -> Bool {
+	switch (left, right) {
+	case let (.External(a, eq), .External(b, _)):
+		return eq(left, right)
+	}
+}