diff --git a/.github/UPSTREAM b/.github/UPSTREAM
index d6d09f6..5547c3b 100644
--- a/.github/UPSTREAM
+++ b/.github/UPSTREAM
@@ -1 +1 @@
-0b45163525d65fb3eda8170c40d6765eb98ddf4d
+43ba4651fe4ce4a4e2ab9d6d60956db78c119d26
diff --git a/lcs/old.go b/lcs/old.go
index 4353da1..7c74b47 100644
--- a/lcs/old.go
+++ b/lcs/old.go
@@ -199,6 +199,7 @@ func (e *editGraph) bdone(D, k int) (bool, lcs) {
 }
 
 // run the backward algorithm, until success or up to the limit on D.
+// (used only by tests)
 func backward(e *editGraph) lcs {
 	e.setBackward(0, 0, e.ux)
 	if ok, ans := e.bdone(0, 0); ok {