interface ReplyNotAllowedForModellingEvent
{
  in void ia();
  out void oa();
  
  behaviour
  {
    on ia: {}
    on optional: {oa; reply();}
  }
  // Simulating ReplyNotAllowedForModellingEvent =>
  //   Reply not allowed for modelling event
  // if the following trace is performed:
  //   oa
}