interface ihello
{
  in void hello();

  behaviour
  {
    void f () {}
    on hello: f();
  }
}

component hello_function
{
  provides ihello h;
  requires iworld w;

  behaviour
  {
    void f() {w.world();}
    on h.hello(): f();
  }
}

interface iworld
{
  in void world();

  behaviour
  {
    on world: {}
  }
}