interface IForward
{
  in void doit();
  out void done();

  behaviour
  {
    bool doing = false;
    on doit: doing = true;
    [doing] on optional: {doing = false; done;}
  }
}


component Forward
{
  provides IForward pp;
  requires IForward rp;

  behaviour
  {
    bool doin = false;
    [!doin] on pp.doit(): {doin = true; rp.doit();}
    [doin] on pp.doit(): {}
    on rp.done(): {doin = false; pp.done();}
  }
}