docs: def verify(path):
          """Verify folder file format
  
          The folder file format is only valid when
          there is only one file format present.
          """
          valid = True
          fifo = SeriesFolder._search_files(path)
          # dataset size
          if len(fifo) == 0:
              valid = False
          # number of different file formats
          fifmts = [ff[1] for ff in fifo]
          if len(set(fifmts)) != 1:
              valid = False
          return valid