public HalfEdge findEdge (Vertex vt, Vertex vh)
	 {
	   HalfEdge he = he0;
	   do
	    { if (he.head() == vh && he.tail() == vt)
	       { return he;
	       }
	      he = he.next;
	    }
	   while (he != he0);
	   return null;
	 }