x = pa[k - 1];
y = x->rtrb_link[1];
x->rtrb_link[1] = y->rtrb_link[0];
y->rtrb_link[0] = x;
pa[k - 2]->rtrb_link[0] = y;

if (x->rtrb_link[1] == NULL)
  {
    x->rtrb_rtag = RTRB_THREAD;
    x->rtrb_link[1] = y;
  }
