Stub diffing in the middle of the graph.

prototype/Doubt/Algorithm.swift

@@ -121,7 +121,15 @@ public enum FreeAlgorithm<A, B> {
 			matrix = Matrix(width: a.count, height: b.count) { i, j in
 				let right = matrix[i + 1, j]
 				let down = matrix[i, j + 1]
+				let diagonal = matrix[i + 1, j + 1]
+
 				let here = Diff.Pure(Patch.Replace(a[i], b[j]))
+
+				if let right = right, down = down, diagonal = diagonal {
+					// nominate the best edge to continue along
+					return Stream.Cons(here, Memo(evaluated: .Nil))
+				}
+
 				// right extent of the edit graph; can only move down
 				if let down = down {
 					return Stream.Cons(here, down)