diff --git a/prototype/Doubt/Patch.swift b/prototype/Doubt/Patch.swift
index fe96595e9..274b79309 100644
--- a/prototype/Doubt/Patch.swift
+++ b/prototype/Doubt/Patch.swift
@@ -53,6 +53,16 @@ extension Patch {
 }
 
 
+// MARK: - Cost calculations
+
+extension Patch {
+	/// Returns a function which computes the size of a `patch` as the sum of the sizes of its terms, as computed by `size`.
+	public static func sum(size: A -> Int)(_ patch: Patch) -> Int {
+		return (patch.state.before.map(size) ?? 0) + (patch.state.after.map(size) ?? 0)
+	}
+}
+
+
 // MARK: - JSON
 
 extension Patch {