!752

class Var {
  new Core core;
  Var next;
  
  Lisp get() {
    ret core.value;
  }
  
  boolean set(Lisp value) {
    ret core.set(value);
  }
  
  void unifyWith(Var v) {
    core = v.core;
  }
}

class Core {
  Lisp value;
  
  boolean set(Lisp val) {
    // existing
    if (value != null)
      ret eq(value, val);
    
    // new
    value = val;
    ret true;
  }
}