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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _parse_this_box(self, fptr, box_id, start, num_bytes):
 """Parse the current box. Parameters fptr : file Open file object, currently points to start of box payload, not the start of the box. box_id : str 4-letter identifier for the current box. start, num_bytes : int Byte offset and length of the current box. Returns ------- Jp2kBox Object corresponding to the current box. """

        try:
            parser = _BOX_WITH_ID[box_id].parse

        except KeyError:
            # We don't recognize the box ID, so create an UnknownBox and be
            # done with it.
            msg = ('Unrecognized box ({box_id}) encountered at byte offset '
                   '{offset}.')
            msg = msg.format(box_id=box_id, offset=fptr.tell() - 8)
            warnings.warn(msg, UserWarning)
            box = UnknownBox(box_id, offset=start, length=num_bytes,
                             longname='Unknown')

            return box

        try:
            box = parser(fptr, start, num_bytes)
        except ValueError as err:
            msg = ("Encountered an unrecoverable ValueError while parsing a "
                   "{box_id} box at byte offset {offset}.  The original error "
                   "message was \"{original_error_message}\".")
            msg = msg.format(box_id=_BOX_WITH_ID[box_id].longname,
                             offset=start,
                             original_error_message=str(err))
            warnings.warn(msg, UserWarning)
            box = UnknownBox(box_id.decode('utf-8'),
                             length=num_bytes,
                             offset=start, longname='Unknown')

        return box