if (x->tavl_tag[1] == TAVL_CHILD)
  y->tavl_link[0] = x->tavl_link[1];
else
  {
    y->tavl_tag[0] = TAVL_THREAD;
    x->tavl_tag[1] = TAVL_CHILD;
  }
x->tavl_link[1] = y;
y->tavl_balance = x->tavl_balance = 0;
