Tell the edge to go out from this vertex.

        Args:
            vertex (Vertex): vertex to go from.

def go_from(self, vertex):
        """
        Tell the edge to go out from this vertex.

        Args:
            vertex (Vertex): vertex to go from.
        """
        if self.vertex_out:
            self.vertex_out.edges_out.remove(self)
        self.vertex_out = vertex
        vertex.edges_out.add(self)