private function mapOperator($operator)
    {
        if (array_key_exists($operator, $this->shortOperators)) {
            $operator = $this->shortOperators[$operator];
        }

        if (!in_array($operator, $this->allowedOperators)) {
            throw new InvalidArgumentException(sprintf(
                'Invalid operator `%s` given. Valid operators are %s.',
                $operator,
                implode(', ', $this->allowedOperators)
            ));
        }

        return $operator;
    }