interface dummy
{
  in void e();
  behaviour
  {
    [true] on e:{}
  }
}

interface imodeling
{
  in void e();
  out void f();
  behaviour
  {
    on optional: f;
    on e: f;
  }
}

component modeling
{
  provides dummy p;
  requires imodeling r;

  behaviour
  {
    on p.e(): r.e();
    on r.f(): {}
  }
}