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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 keys(self):
 """ Returns all row keys :raise NotImplementedError: if all rows aren't keyed :return: all row keys """

        if len(self._row_name_list) != len(self._rows):
            raise NotImplementedError("You can't get row keys for a FVM that doesn't have all rows keyed")

        return self.row_names()