interface Top
{
  in void unguarded();
  in void e();
  out void f();

  behaviour
  {
    on unguarded: {}
    on e: {}
    on optional: f;
  }
}

interface Bottom
{
  in void e();
  out void f();

  behaviour
  {
    bool c = false;

    [!c]
    {
      on e: {c = true;}
    }
    [c]
    {
      on e: illegal;
      on inevitable: {c = false; f;}
    }
  }
}

component GuardedRequiredIllegal
{
  provides Top t;
  requires Bottom b;

  behaviour
  {
    bool c = false;

    on t.unguarded(): {}

    [!c]
    {
      on t.e(): {c = true; b.e();}
      on b.f(): illegal;
    }
    [c]
    {
      on t.e(): {}
      on b.f(): {c = false;}
    }
  }
}