ill_value=999999), array([0, 1, 4, 2]))
    >>> ma.unique(masked_a, return_inverse=True)
    (masked_array(data=[1, 2, 3, --],
                mask=[False, False, False,  True],
        fill_value=999999), array([0, 1, 3, 1, 2]))
    >>> ma.unique(masked_a, return_index=True, return_inverse=True)
    (masked_array(data=[1, 2, 3, --],
                mask=[False, False, False,  True],
        fill_value=999999), array([0, 1, 4, 2]), array([0, 1, 3, 1, 2]))
    )