quoting not-in-values

    /**
     * Creates a NOT IN() expression with the given arguments.
     *
     * @param string $x Field in string format to be restricted by NOT IN() function
     * @param mixed $y Argument to be used in NOT IN() function.
     * @return Expr\Func
     */
    public function notIn($x, $y)
    {
        if (is_array($y)) {
            foreach ($y as &$literal) { 
                if ( ! ($literal instanceof Expr\Literal)) { 
                    $literal = $this->_quoteLiteral($literal);
                }
            }
        }
        return new Expr\Func($x . ' NOT IN', (array) $y);
    }