code: 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;
      }
docs: Finds the half-edge within this face which has tail <code>vt</code> and
  head <code>vh</code>.
  
  @param vt
  tail point
  @param vh
  head point
  @return the half-edge, or null if none is found.