public static String getVariableThatBelongsInUsefulVariableSet(
			Grammar grammar, Set set) {
		String[] variables = grammar.getVariables();
		for (int k = 0; k < variables.length; k++) {
			if (belongsInUsefulVariableSet(variables[k], grammar, set)
					&& !set.contains(variables[k]))
				return variables[k];
		}
		return null;
	}