code: def _build_frange_part(start, stop, stride, zfill=0):
          
          if stop is None:
              return ''
          pad_start = pad(start, zfill)
          pad_stop = pad(stop, zfill)
          if stride is None or start == stop:
              return '{0}'.format(pad_start)
          elif abs(stride) == 1:
              return '{0}-{1}'.format(pad_start, pad_stop)
          else:
              return '{0}-{1}x{2}'.format(pad_start, pad_stop, stride)
docs: Private method: builds a proper and padded
          frame range string.
  
          Args:
              start (int): first frame
              stop (int or None): last frame
              stride (int or None): increment
              zfill (int): width for zero padding
  
          Returns:
              str: