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;
  }
