if (x->rtavl_rtag == RTAVL_CHILD)
  y->rtavl_link[0] = x->rtavl_link[1];
else
  {
    y->rtavl_link[0] = NULL;
    x->rtavl_rtag = RTAVL_CHILD;
  }
x->rtavl_link[1] = y;
y->rtavl_balance = x->rtavl_balance = 0;
