def sum_in(self):
        """Return sum of all edges that point to vertex.

        >>> g = Graph(VertexType=WVertex)
        >>> g.add(1, [1, 2, 3])
        >>> g.add(4, 1, 3)
        >>> g[1].sum_in(), g[3].sum_in(), g[4].sum_in()
        (4, 1, 0)
        """
        g, t = self._graph, self._id
        sum = 0
        for h in self.in_vertices():
            sum += g[h][t]
        return sum
Return sum of all edges that point to vertex.

        >>> g = Graph(VertexType=WVertex)
        >>> g.add(1, [1, 2, 3])
        >>> g.add(4, 1, 3)
        >>> g[1].sum_in(), g[3].sum_in(), g[4].sum_in()
        (4, 1, 0)
        
recipes/Python/578615_graph/recipe-578615.py