<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def parse(cls, fptr, offset, length):
 """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. """

        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)