concept UserName {
  S name;
  bool usable;
}

concept Line {
  S text;
  new Ref<LineIdea> idea;

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

concept UserLine extends Line {
  *() {}
  *(S text) { super(text); }
}

concept BotLine extends Line {
  *() {}
  *(S text, LineIdea idea) {
    super(text);
    this.idea.set(idea);
  }
}

concept LineIdea {}

concept LError extends LineIdea {
  S stackTrace;
  
  *() {}
  *(Throwable e) { stackTrace = getStackTrace(e); change(); }
}

concept LGreet extends LineIdea {
  new Ref<UserName> userName;
  
  *() {}
  *(UserName u) { userName.set(u); }
}

concept LWhatsYourName extends LineIdea {}

concept LIDontUnderstand extends LineIdea {}

concept Dialog {
  new RefL<Line> lines;
}

static Dialog dialog() {
  ret unique(Dialog);
}