struct trb_node *y = pa[k - 2]->trb_link[1];
if (pa[k - 2]->trb_tag[1] == TRB_CHILD && y->trb_color == TRB_RED)
  {
    pa[k - 1]->trb_color = y->trb_color = TRB_BLACK;
    pa[k - 2]->trb_color = TRB_RED;
    k -= 2;
  }
else
  {
    struct trb_node *x;

    if (da[k - 1] == 0)
      y = pa[k - 1];
    else
      {
        x = pa[k - 1];
        y = x->trb_link[1];
        x->trb_link[1] = y->trb_link[0];
        y->trb_link[0] = x;
        pa[k - 2]->trb_link[0] = y;

        if (y->trb_tag[0] == TRB_THREAD)
          {
            y->trb_tag[0] = TRB_CHILD;
            x->trb_tag[1] = TRB_THREAD;
            x->trb_link[1] = y;
          }
      }

    x = pa[k - 2];
    x->trb_color = TRB_RED;
    y->trb_color = TRB_BLACK;

    x->trb_link[0] = y->trb_link[1];
    y->trb_link[1] = x;
    pa[k - 3]->trb_link[da[k - 3]] = y;

    if (y->trb_tag[1] == TRB_THREAD)
      {
        y->trb_tag[1] = TRB_CHILD;
        x->trb_tag[0] = TRB_THREAD;
        x->trb_link[0] = y;
      }
    break;
  }
