How to use the h5pyd.special_dtype function in h5pyd

else:
                    tgt_dt = h5py.special_dtype(ref=h5py.RegionReference)
            else:
                msg = "Unexpected ref type: {}".format(srcdt)
                logging.error(msg)
                raise TypeError(msg)
        elif srcdt.metadata and 'vlen' in srcdt.metadata:
            src_vlen = srcdt.metadata['vlen']
            if isinstance(src_vlen, np.dtype):
                tgt_base = convert_dtype(src_vlen, ctx)
            else:
                tgt_base = src_vlen
            if is_h5py(ctx['fout']):
                tgt_dt = h5py.special_dtype(vlen=tgt_base)
            else:
                tgt_dt = h5pyd.special_dtype(vlen=tgt_base)
        else:
            tgt_dt = srcdt
    return tgt_dt