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;
  }
