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

if (y->rtrb_rtag == RTRB_THREAD)
  {
    y->rtrb_rtag = RTRB_CHILD;
    x->rtrb_link[0] = NULL;
  }
