<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_all_keys(self):
 """Get all keys indexed. :return: All keys :rtype: list(str) """

        all_keys = []
        for keys in self._index.values():
            all_keys.extend(keys)
        return all_keys