def __delitem__(self, tail):
        """Removes tail from self and self._id from tail.reverse.

        >>> class myvertex(ReverseEdgeMixin, WVertex):
        ...    pass
        >>> g = Graph(VertexType=myvertex)
        >>> g[1][2] = 3
        >>> g[2].reverse
        {1: 3}
        >>> del g[1][2]
        >>> g[2].reverse
        {}
        """
        if tail in self._graph:
            del self._graph[tail].reverse[self._id]  #creates tail vertex so check if tail in graph first
        super(ReverseEdgeMixin, self).__delitem__(tail)
Removes tail from self and self._id from tail.reverse.

        >>> class myvertex(ReverseEdgeMixin, WVertex):
        ...    pass
        >>> g = Graph(VertexType=myvertex)
        >>> g[1][2] = 3
        >>> g[2].reverse
        {1: 3}
        >>> del g[1][2]
        >>> g[2].reverse
        {}
        
recipes/Python/578615_graph/recipe-578615.py