Int? parseInt(Str from) {
	if (from.isInt()) {
		return from.toInt();
	} else {
		return null;
	}
}

Int? addIf(Int? a, Int? b) {
	if (a) {
		if (b) {
			return a + b;
		}
	}
	return null;
}

Int? addUnless(Int? a, Int? b) {
	unless (a)
		return null;
	unless (b)
		return null;

	return a + b;
}

void maybe() {
	Int? result = addUnless(parseInt("10"), parseInt("20"));
	print(result.toS);
}
