diff -r b141080e70c5 -r 33943add5d65 mercurial/revset.py
--- a/mercurial/revset.py	Fri Mar 14 10:59:51 2014 -0700
+++ b/mercurial/revset.py	Fri Mar 14 10:55:03 2014 -0700
@@ -2254,6 +2254,10 @@
     revset
     """
     def __init__(self, subset, condition=lambda x: True):
+        """
+        condition: a function that decide whether a revision in the subset
+                   belongs to the revset or not.
+        """
         self._subset = subset
         self._condition = condition
         self._cache = {}