concept UserLine {
  S text;
  
  *() {}
  *(S *text) { change(); }
}

concept UserAnswer {
  new Ref<UserLine> userLine;
  S text;
  bool solvable;
}

concept SystemAnswer {
  new Ref<UserLine> userLine;
  S text;
}

concept SCDefaults {
  bool solvable;
}