def METHOD_NAME(self):
    u = case1()
    fgs = u.atoms.fragments
    assert fgs is u.atoms._cache['fragments']
    assert u.atoms._cache_key in u._cache['_valid']['fragments']
    u.add_bonds((fgs[0][-1] + fgs[1][0],))  # should trigger invalidation
    assert 'fragments' not in u._cache['_valid']
    assert len(fgs) > len(u.atoms.fragments)  # recomputed