   if (crossing.length === 1 && !start && !end && edgesAtStart.length === 0 && edgesAtEnd.length === 0) {
            topo.remEdgeNewFace(crossing[0]);
            start = crossing[0].start;
            if (start.face) {
              topo.removeIsoNode(start);
            }
            end = crossing[0].end;
            if (end.face) {
              topo.removeIsoNode(end);
            }
            return;
          }
          if (!start) {
            start = createNode(topo, startCoord);
            ed