diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index cef03f8..f640661 100644
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -10,6 +10,7 @@
  * The tree can easily be walked from child to parent, but not in the
  * opposite direction.
  *
+ * This code is based on xact.c, but the robustness requirements
  * are completely different from pg_xact, because we only need to remember
  * pg_subtrans information for currently-open transactions.  Thus, there is
  * no need to preserve data over a crash and restart.