docs: def clean_dict(data):
      """Remove None-valued keys from a dictionary, recursively."""
      if is_mapping(data):
          out = {}
          for k, v in data.items():
              if v is not None:
                  out[k] = clean_dict(v)
          return out
      elif is_sequence(data):
          return [clean_dict(d) for d in data if d is not None]
      return data