If performance is crucial, we can keep the index as a first parameter for the `file_out` function and we can change the definition of the `file_out` modparam: ``` modparam("file_out", "base_filename", "0:accounting") modparam("file_out", "base_filename", "1:missed_calls") ``` Populate slot `0` with `accounting` and slot `1` with `missed_calls`. If a slot is re-used, throw an error and exit. If a slot is bigger then `9`, throw an error and exit.