public Collection<?> collection(String string) {
		parseCheck();
		Option<?> opt = options.get(string);
		if (opt != null) {
			if (opt instanceof CollectionOption<?, ?>)
				return (Collection<?>) opt.value();
			throw new RuntimeException("--" + opt.name
					+ " is not a repeatable option");
		}
		throw new RuntimeException("unknown option --" + string);
	}