add explicit imports in schema declaration

schema.py

@@ -3,9 +3,13 @@
 :copyright: 2003-2009 LOGILAB S.A. (Paris, FRANCE), all rights reserved.
 :contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
 """
+from yams.buildobjs import (MetaUserEntityType, MetaUserRelationType,
+                            SubjectRelation, RichString)
+
+from cubicweb.schema import RRQLExpression
 
 class Comment(MetaUserEntityType):
     """a comment is a reply about another entity"""
     content = RichString(required=True, fulltextindexed=True)
 
     comments = SubjectRelation('Comment', cardinality='1*', composite='object')
@@ -6,11 +10,9 @@
 
 class Comment(MetaUserEntityType):
     """a comment is a reply about another entity"""
     content = RichString(required=True, fulltextindexed=True)
 
     comments = SubjectRelation('Comment', cardinality='1*', composite='object')
-    #comments = SubjectRelation('*')
-
 
 class comments(MetaUserRelationType):
     permissions = {