<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_index(self, prefix=''):
 """ Retrieve the absolute path to an index, according to `prefix`. Args: prefix: str, the desired prefix or `None`. Returns: str: An absolute path, or `None` """

        if prefix:
            prefixed = '%s_index' % prefix
        else:
            prefixed = 'index'

        if prefixed in self.__cli and self.__cli[prefixed]:
            index = self.__cli.get(prefixed)
            from_conf = False
        else:
            index = self.__config.get(prefixed)
            from_conf = True

        return self.__abspath(index, from_conf)