/**
  * @return All vertices with an edge going to v
  */
 public IEnumerable <Vertex> origins(Vertex v)
 {
     return(v.origins());
 }
	/**
	 * @return All vertices with an edge going to v
	 */
	public IEnumerable<Vertex> origins(Vertex v)
	{
		return v.origins();
	}