Create a DirectedGraph from a collection of vertices and
        a mapping giving the vertices that each vertex is connected to.

def from_out_edges(cls, vertices, edge_mapper):
        """
        Create a DirectedGraph from a collection of vertices and
        a mapping giving the vertices that each vertex is connected to.

        """
        vertices = set(vertices)
        edges = set()
        heads = {}
        tails = {}

        # Number the edges arbitrarily.
        edge_identifier = itertools.count()
        for tail in vertices:
            for head in edge_mapper[tail]:
                edge = next(edge_identifier)
                edges.add(edge)
                heads[edge] = head
                tails[edge] = tail

        return cls._raw(
            vertices=vertices,
            edges=edges,
            heads=heads,
            tails=tails,
        )