def _repack(h5file):
    """
    Repack archive to remove freespace.
               
    Returns
    -------
    file : h5py File or None
        If the input is a h5py.File then a h5py File instance of the
        repacked archive is returned. The input File instance will no longer
        be useable. 
    """
    f1, opened = _openfile(h5file) 
    filename1 = f1.filename
    filename2 = filename1 + '_repack_tmp'
    f2 = h5py.File(filename2, 'w')
    for key in f1.keys():
        # print 'copying', key
        f1.copy(key, f2)
    f1.close()
    f2.close()
    filename_tmp = filename1 + '_repack_rename_tmp'
    os.rename(filename1, filename_tmp)
    os.rename(filename2, filename1) 
    if opened:
        f = None  
    else:
        f = h5py.File(filename1)
    os.remove(filename_tmp)
    return f