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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _fetch_events_files_on_disk(self):
 """Obtains the names of debugger-related events files within the directory. Returns: The names of the debugger-related events files written to disk. The names are sorted in increasing events file index. """

    all_files = tf.io.gfile.listdir(self._events_directory)
    relevant_files = [
        file_name for file_name in all_files
        if _DEBUGGER_EVENTS_FILE_NAME_REGEX.match(file_name)
    ]
    return sorted(relevant_files, key=self._obtain_file_index)