interface I {
  enum Res { OK, NOK };
  in Res e();

  behaviour {
     enum State { Idle, Error };
     State s = State.Idle;
     [s.Idle] {
       on e: reply(Res.OK);
       on optional: s = State.Error;
     }  
     [s.Error] {
       on e: reply(Res.NOK);
       on optional: s = State.Idle;
     }  
  }
}

component silentoptional {
  provides I p;
  behaviour {
     on p.e(): reply(I.Res.NOK);
  }
}