typo - existing edge check


  def add_edge(self, eid, frm, to, elb):
        """Add an edge to the graph."""
        if (frm is self.vertices and
                to in self.vertices and
                to in self.vertices[frm].edges):
            return self



        if (frm is self.vertices and 

--> to be change to 

        if (frm in self.vertices and 

?
