code: def all_sample_keys(self) -> List[str]:
          
          all_keys: List[str] = []
          for key in chain.from_iterable([sample.keys() for sample in self]):
              if key not in all_keys:
                  all_keys.append(key)
          return all_keys
docs: Return the unique keys of all samples in this :class:`SampleSheet`.
  
          The keys are discovered first by the order of samples and second by
          the order of keys upon those samples.