protected function getOperator(string $operator): callable
    {
        if (!$this->hasOperator($operator)) {
            throw new OperatorNotFoundException($operator, sprintf('Operator "%s" does not exist.', $operator));
        }

        return $this->operators[$operator];
    }
protected function getOperator(string $operator): callable
    {
        if (!$this->hasOperator($operator)) {
            throw new OperatorNotFoundException($operator, sprintf('Operator "%s" does not exist.', $operator));
        }

        return $this->operators[$operator];
    }
protected function getOperator ( string $ operator ) : callable { if ( ! $ this -> hasOperator ( $ operator ) ) { throw new OperatorNotFoundException ( $ operator , sprintf ( 'Operator "%s" does not exist.' , $ operator ) ) ; } return $ this -> operators [ $ operator ] ; }
Get an operator.

@param string $operator The operator's name.

@throws OperatorNotFoundException
Get an operator .