rev_top_sort

    def _rev_top_sort (self, dag):
        """Simple reverse top sorter given dag
        
        dag: a dictionary with node:(neighbors)
        returns a list of nodes in reversed topological order.
        
        Used to determine update sequence.
        """
        ret_list = []
        visited = []

        def _visit (node, deps):
            if node in deps:
                raise ValueError, "Given dag has a cycle."
            if node not in visited:
                visited.append(node)
                deps = deps + [node]
                map(lambda n: _visit(n, deps), dag[node])
                ret_list.append(node)

        map(lambda n: _visit(n, []), dag.keys())
        return ret_list