Tell the edge to go into this vertex.

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

def go_in(self, vertex):
        """
        Tell the edge to go into this vertex.

        Args:
            vertex (Vertex): vertex to go into.
        """
        if self.vertex_in:
            self.vertex_in.edges_in.remove(self)
        self.vertex_in = vertex
        vertex.edges_in.add(self)