def METHOD_NAME():
    '''Test the _add_arg utility method.'''
    field_meta_arg = FieldArgMetadata("GH_REAL", "GH_WRITE", "W0")
    scalar_meta_arg = ScalarArgMetadata("GH_REAL", "GH_READ")
    metadata = LFRicKernelMetadata(
        operates_on="cell_column", meta_args=[field_meta_arg, scalar_meta_arg])
    cls = ArgIndexToMetadataIndex
    cls._initialise(None)
    cls._metadata = metadata
    # call add_arg
    cls._add_arg(field_meta_arg)
    assert len(cls._info) == 1
    # pylint: disable=unsubscriptable-object
    assert cls._info[0] == 0
    assert cls._index == 1
    # call add_arg again
    cls._add_arg(scalar_meta_arg)
    assert len(cls._info) == 2
    assert cls._info[0] == 0
    assert cls._info[1] == 1
    assert cls._index == 2