def METHOD_NAME(self, kwargs, expected):
    """Test the structure of the returned values."""
    ws = two_fluid(**kwargs)
    assert isinstance(ws, dict)
    assert len({"acoustic_mode", "alfven_mode", "fast_mode"} - set(ws.keys())) == 0
    for val in ws.values():
        assert isinstance(val, u.Quantity)
        assert val.unit == u.rad / u.s
        assert val.shape == expected["shape"]