primitive pres_2(in i, out o) {
  sync {
    put(o, get(i));
  }
}
primitive together(in ia, in ib, out oa, out ob){
  while(true) sync {
    if(fires(ia)) {
      put(oa, get(ia));
      put(ob, get(ib));
    }
  }	
}

primitive alt_round_merger(in a, in b, out c){
  while(true) {
    sync { put(c, get(a)); }
    sync { put(c, get(b)); }
  }	
}