code: def parse(cls, fptr, offset, length):
          
          num_bytes = offset + length - fptr.tell()
          read_buffer = fptr.read(num_bytes)
          mask_length, = struct.unpack_from('>B', read_buffer, offset=0)
  
          # Fully Understands Aspect Mask
          # Decodes Completely Mask
          fuam = dcm = standard_flag = standard_mask = []
          vendor_feature = vendor_mask = []
  
          # The mask length tells us the format string to use when unpacking
          # from the buffer read from file.
          try:
              mask_format = {1: 'B', 2: 'H', 4: 'I', 8: 'Q'}[mask_length]
              fuam, dcm = struct.unpack_from('>' + mask_format * 2, read_buffer,
                                             offset=1)
              std_flg_offset = 1 + 2 * mask_length
              data = _parse_standard_flag(read_buffer[std_flg_offset:],
                                          mask_length)
              standard_flag, standard_mask = data
  
              nflags = len(standard_flag)
              vndr_offset = 1 + 2 * mask_length + 2 + (2 + mask_length) * nflags
              data = _parse_vendor_features(read_buffer[vndr_offset:],
                                            mask_length)
              vendor_feature, vendor_mask = data
  
          except KeyError:
              msg = ('The ReaderRequirements box (rreq) has a mask length of '
                     '{length} bytes, but only values of 1, 2, 4, or 8 are '
                     'supported.  The box contents will not be interpreted.')
              warnings.warn(msg.format(length=mask_length), UserWarning)
  
          return cls(fuam, dcm, standard_flag, standard_mask,
                     vendor_feature, vendor_mask,
                     length=length, offset=offset)
docs: Parse reader requirements box.
  
          Parameters
          ----------
          fptr : file
              Open file object.
          offset : int
              Start position of box in bytes.
          length : int
              Length of the box in bytes.
  
          Returns
          -------
          ReaderRequirementsBox
              Instance of the current reader requirements box.