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

ref_start, _, _ = self.ref_indices[ref_id]
                self._io.seek(ref_start)
            except KeyError:
                raise KeyError('Reference is not found in header')
        ref_start = self._io.tell()

        if not idx:
            assert ref_start == self.ref_indices[ref_id].start_offset, 'ref not properly aligned'

        n_bins = unpack_int32L(self._io.read(4))[0]
        bins = self.get_bins(n_bins, ref_id, idx)

        self._last_pos = self._io.tell()

        if idx:
            return bai.RefIdx(ref_start, self._last_pos, n_bins)
        else:
            return RefCsi(bins, ref_id)