!752

static class Var extends Lisp {
  Lisp instance;
  
  *(S name) {
    super(name);
    instance = this;
  }
}

static boolean unify(Lisp thiz, Lisp t) {
  if (thiz instanceof Var) {
    Var v = cast thiz;
    if (v.instance != v)
      return unify(v.instance, t);
    //Trail::Push(v);
    v.instance = t;
    return true;
  }
  
  return unify2(t, thiz);
}

static boolean unify2(Lisp thiz, Lisp t) { 
  if (thiz instanceof Var)
    return unify(thiz, t);
 
  int arity = thiz.size();
  if (neq(thiz.head, t.head) || arity != t.size())
    return false;
  for (int i = 0; i < arity; i++)
    if (!unify(thiz.get(i), t.get(i)))
      return false;
  return true;
}

p {
  Var x = new Var("X");
  print(x + " = " + x.instance);
  print(unify(x, lisp("hello")));
  print(x + " = " + x.instance);
}