Test cases for this functionality are:
```
    #Mix of multilabel-indicator and multilabel-sequences
    assert_array_equal(unique_labels([["a", "b"], ["c"]], np.ones((3, 3))),
                       ["a", "b", "c"])
    assert_raises(ValueError, unique_labels, [["a", "b"], ["c"]],
                  np.ones((3, 4)))
    assert_raises(ValueError, unique_labels, [["a", "b"], ["c", 'd']],
                  np.ones((3, 3)))
```