diff --git a/prototype/doubt-difftool/main.swift b/prototype/doubt-difftool/main.swift
index fd9e3313b..893190e08 100644
--- a/prototype/doubt-difftool/main.swift
+++ b/prototype/doubt-difftool/main.swift
@@ -156,7 +156,7 @@ func refineLeafReplacement(aString: String, _ bString: String)(_ patch: Patch<Te
 	case let .Replace(.Unroll(aExtract, .Leaf), .Unroll(bExtract, .Leaf)):
 		let a = aString.utf16[aExtract.range].enumerate().map { Term(Info(range: (aExtract.range.startIndex + 0).range, categories: aExtract.categories), .Leaf(String($1))) }
 		let b = bString.utf16[bExtract.range].enumerate().map { Term(Info(range: (bExtract.range.startIndex + 0).range, categories: bExtract.categories), .Leaf(String($1))) }
-		return .Roll((aExtract, bExtract), .Indexed(SES(a, b, cost: const(1), recur: const(nil))))
+		return .Roll((aExtract, bExtract), .Indexed(SES(a, b, cost: const(1), recur: { $0 == $1 ? Term.zip($0, $1).map(Diff.init) : Diff.Replace($0, $1) })))
 	default:
 		return .Pure(patch)
 	}