code: def parse(cls, fptr, offset, length):
          
          num_bytes = offset + length - fptr.tell()
          read_buffer = fptr.read(num_bytes)
          num_fragments, = struct.unpack_from('>H', read_buffer, offset=0)
  
          lst = struct.unpack_from('>' + 'QIH' * num_fragments,
                                   read_buffer,
                                   offset=2)
          frag_offset = lst[0::3]
          frag_len = lst[1::3]
          data_reference = lst[2::3]
          return cls(frag_offset, frag_len, data_reference,
                     length=length, offset=offset)
docs: Parse JPX free box.
  
          Parameters
          ----------
          f : file
              Open file object.
          offset : int
              Start position of box in bytes.
          length : int
              Length of the box in bytes.
  
          Returns
          -------
          FragmentListBox
              Instance of the current fragment list box.