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