def METHOD_NAME(self):
    r"""
    Return a subset of the labels that "appear" in this structure.
    EXAMPLES::
        sage: P = species.SubsetSpecies()
        sage: S = P.structures(["a", "b", "c"])
        sage: [s.label_subset() for s in S]
        [[], ['a'], ['b'], ['c'], ['a', 'b'], ['a', 'c'], ['b', 'c'], ['a', 'b', 'c']]
    """
    return [self._relabel(i) for i in self._list]