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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 remap_index_fn(ref_file):
 """Map sequence references to snap reference directory, using standard layout. """

    snap_dir = os.path.join(os.path.dirname(ref_file), os.pardir, "snap")
    assert os.path.exists(snap_dir) and os.path.isdir(snap_dir), snap_dir
    return snap_dir