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update doc

diff --git a/states.py b/states.py
--- a/states.py
+++ b/states.py
@@ -51,6 +51,9 @@ class state(object):
 
     @util.propertycache
     def trackheads(self):
+        """Do we need to track heads of changeset in this state ?
+
+        We don't need to track heads for the last state as this is repos heads"""
         return self.next is not None
 
     def __cmp__(self, other):
@@ -58,6 +61,7 @@ class state(object):
 
     @util.propertycache
     def _revsetheads(self):
+        """function to be used by revset to finds heads of this states"""
         assert self.trackheads
         def revsetheads(repo, subset, x):
             args = revset.getargs(x, 0, 0, 'publicheads takes no arguments')
@@ -68,6 +72,7 @@ class state(object):
 
     @util.propertycache
     def headssymbol(self):
+        """name of the revset symbols"""
         if self.trackheads:
             return "%sheads" % self.name
         else: