diff --git a/src/fix.py b/src/fix.py
index d322231b4..e6d8c98f4 100644
--- a/src/fix.py
+++ b/src/fix.py
@@ -259,5 +259,8 @@ def partition(p, L):
         else:
             no.append(elt)
     return (yes, no)
+
+def flip((x, y)):
+    return (y, x)
     
 # vim:set shiftwidth=4 tabstop=8 expandtab textwidth=78: