public static function normaliseOperator($operator)
    {
        switch ($operator) {
            case 'add':
            case 'addition':
                $operator = '+';
                break;
            case 'sub':
            case 'subtract':
                $operator = '-';
                break;
            case 'mul':
            case 'multiply':
                $operator = '*';
                break;
            case 'div':
            case 'divide':
                $operator = '/';
                break;
            case 'mod':
            case 'modulus':
                $operator = '%';
                break;
            case 'comp':
            case 'compare':
                $operator = '=';
                break;
        }
        return $operator;
    }