comparison of **node objects** based on the ``"branch"`` attribute of 
        their **topological nodes**.

def cmp_branch(self, node1, node2):
        """
        comparison of **node objects** based on the ``"branch"`` attribute of 
        their **topological nodes**.
        
        """
        # note reverse
        return cmp(getattr(self[node2], "branch", None), \
                   getattr(self[node1], "branch", None))