def METHOD_NAME(self):
    c1 = Coordinates([[0, 1], [0, 1]], dims=["lat", "lon"])
    c2 = Coordinates([[10, 11], [10, 11]], dims=["lat", "lon"])
    c3 = Coordinates(["2018-01-01"], dims=["time"])
    g1 = GroupCoordinates([c1])
    g2 = GroupCoordinates([c2])
    g3 = GroupCoordinates([c3])
    g1 += g2
    with pytest.raises(ValueError):
        g1 += g3
    with pytest.raises(TypeError):
        g1 += c1
    assert len(g1) == 2
    assert g1._items[0] is c1
    assert g1._items[1] is c2
    assert len(g2) == 1
    assert g2._items[0] is c2