<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def file_del(self, key, paths=None):
 """ Deregister a task for file event changes. If paths is None, all paths assoicated with the task will be deregistered. """

        if paths is None:
            paths = []
            for path in self._file_event_map:
                if key in self._file_event_map[path]:
                    paths.append(path)
        elif not isinstance(paths, list):
            paths = [paths]
        for path in paths:
            if key in self._file_event_map[path]:
                del self._file_event_map[path][key]
            if path in self._file_event_map and not self._file_event_map[path]:
                self._watch_files.remove(path)
                del self._file_event_map[path]