How to use the bamnostic.bai.Unmapped function in bamnostic

Returns:
            bins (None | dict): None if just indexing the index file or a dictionary
                                of `bin_id: chunks` pairs
        Raises:
            AssertionError (Exception): if bin 37450 does not contain 2 chunks exactly
        """
        bins = None if idx else {}

        for b in range(n_bins):
            bin_id, n_chunks = unpack_bid_nchunk(self._io.read(8))

            if idx:
                if bin_id == self._UNMAP_BIN:
                    assert n_chunks == 2, 'Bin 3740 is supposed to have 2 chunks. This has {}'.format(n_chunks)
                    unmapped = Unmapped(*unpack_unmapped(self._io.read(32)))
                    self.unmapped[ref_id] = unmapped
                else:
                    if not n_chunks == 0:
                        self._io.seek(16 * n_chunks, 1)  # 16 = struct.calcsize('<2Q')
            else:
                chunks = self.get_chunks(n_chunks)
                bins[bin_id] = chunks
        else:
            return bins