private Value plus(String arg1, String arg2) {
		final Value v1 = evaluate(arg1);
		final Value v2 = evaluate(arg2);
		if (v1 instanceof ValueInt && v2 instanceof ValueInt) {
			return new ValueInt(((ValueInt) v1).getValue() + ((ValueInt) v2).getValue());
		}
		if (v1 instanceof ValueTime && v2 instanceof ValueInt) {
			final int nb = ((ValueInt) v2).getValue();
			TimeElement t = ((ValueTime) v1).getValue();
			if (nb > 0) {
				for (int i = 0; i < nb; i++) {
					t = timeline.next(t);
				}
			}
			if (nb < 0) {
				for (int i = 0; i < -nb; i++) {
					t = timeline.previous(t);
				}
			}
			return new ValueTime(t);
		}
		throw new UnsupportedOperationException();
	}