public function AliasIdentificationVariable()
    {
        $this->match(Lexer::T_IDENTIFIER);

        $aliasIdentVariable = $this->lexer->token['value'];
        $exists             = isset($this->queryComponents[$aliasIdentVariable]);

        if ($exists) {
            $this->semanticalError(sprintf("'%s' is already defined.", $aliasIdentVariable), $this->lexer->token);
        }

        return $aliasIdentVariable;
    }