// a map of assignments that can be rolled back to some state
static class SNLMatches {
  new HashMap<S, Lisp> map;
  new L<S> log;
  
  boolean put(S key, Lisp val) {
    if (map.containsKey(key))
      // existing
      ret eq(map.get(key), val);
    
    // new
    map.put(key, val);
    log.add(key);
    ret true;
  }
  
  Lisp get(S key) {
    ret map.get(key);
  }
  
  S raw(S key) {
    Lisp x = get(key);
    if (x == null) fail("var not set: " + key);
    ret x.raw();
  }
  
  boolean containsKey(S key) {
    ret map.containsKey(key);
  }
  
  int save() {
    ret l(log);
  }
  
  void restore(int i) {
    while (l(log) > i) {
      S key = log.get(l(log)-1);
      map.remove(key);
      log.remove(l(log)-1);
    }
  }
  
  boolean addAll(SNLMatches m) {
    for (S var : m.keySet())
      if (!put(var, m.get(var)))
        ret false;
    ret true;
  }
  
  Set<S> keySet() {
    ret map.keySet();
  }
}