sclass ErrorCounter {
  LPair<O, Throwable> errors;
  
  void run(O info, Runnable r) {
    try {
      r.run();
    } catch print e {
      errors.add(pair(info, e));
    }
  }
  
  void print() {
    if (empty(errors)) print("No errors");
    else print(n2(errors, "error") + in " + pairsA(errors));
  }
}