Returns the maximum rank for each **topological node** in the 
        ``DictGraph``. The rank of a node is defined as the number of edges 
        between the node and a node which has rank 0. A **topological node** 
        has rank 0 if it has no incoming edges.

def node_rank(self):
        """
        Returns the maximum rank for each **topological node** in the 
        ``DictGraph``. The rank of a node is defined as the number of edges 
        between the node and a node which has rank 0. A **topological node** 
        has rank 0 if it has no incoming edges.
        
        """
        nodes = self.postorder()
        node_rank = {}
        for node in nodes:
            max_rank = 0
            for child in self[node].nodes():
                some_rank = node_rank[child] + 1
                max_rank = max(max_rank, some_rank)
            node_rank[node] = max_rank
        return node_rank