primitive forward(in i, out o) {
	while(true) synchronous {
		put(o, get(i));
	}
}
primitive sync(in i, out o) {
	while(true) synchronous {
		if(fires(i)) {
			put(o, get(i));
		}
	}
}

primitive sync_two(in ia, in ib, out oa, out ob) {
	while(true) synchronous {
		if (fires(ia)) {
			put(oa, get(ia));
			put(ob, get(ib));
		}
	}
}

composite bob3(in i, out o) {
	new sync(i, o);
}
composite bob5(in i, out o) {
	new sync(i, o);
}

primitive xor_three(in ai, out ao, in bi, out bo, in ci, out co) {
	synchronous {
		if      (fires(ai)) put(ao, get(ai));
		else if (fires(bi)) put(bo, get(bi));
		else                put(co, get(ci)); 
	}
}