diff -r 85c634ff395a -r 70809438c644 mercurial/transaction.py
--- a/mercurial/transaction.py	Wed Nov 05 10:05:38 2014 +0000
+++ b/mercurial/transaction.py	Wed Nov 05 13:00:48 2014 +0000
@@ -142,6 +142,7 @@
 
     @active
     def add(self, file, offset, data=None):
+        """record the state of an append-only file before update"""
         if file in self.map or file in self._backupmap:
             return
         if self._queue: