public function parseString($string)
    {
        $tokens = $this->tokenize($string);

        $tokens = array_filter(
            $tokens,
            function ($token) {
                return !($token['type'] === TransformationProcessor::T_WHITESPACE ||
                          $token['type'] === TransformationProcessor::T_COMMENT);
            }
        );

        $ast = array();
        $section = null;
        while ($token = array_shift($tokens)) {
            if ($token['type'] === TransformationProcessor::T_SECTION) {
                $section = $token['data']['section'];
                continue;
            }

            if ($section === null) {
                throw new RuntimeException('Expected section.');
            }

            $ast[$section][] = $token;
        }

        return $ast;
    }