* NEWS: Mention the change.
* python/spot/__init__.py: Add _repr_latex_ for twa_word, and
remove __repr__ and __str__ for atomic_prop_set.
* python/spot/impl.i: Implement __repr__ and __str__ for
atomic_prop_set. Fix __repr__ for trival, acc_code, acc_cond,
mark_t. Remove __repr__ for twa_run and twa_word.
* tests/python/acc_cond.ipynb, tests/python/accparse.ipynb,
tests/python/atva16-fig2a.ipynb, tests/python/automata.ipynb,
tests/python/bdditer.py, tests/python/contains.ipynb,
tests/python/gen.ipynb, tests/python/highlighting.ipynb,
tests/python/ltlsimple.py, tests/python/ltsmin-dve.ipynb,
tests/python/product.ipynb, tests/python/relabel.py,
tests/python/satmin.ipynb tests/python/stutter-inv.ipynb,
tests/python/word.ipynb: Adjust test cases.
* tests/python/formulas.ipynb: Add test for atomic_prop_set.
This prevents an exception from being raised if NNF is not performed
on Boolean properties and implication-based checks are used.
* NEWS: Mention the issue.
* spot/tl/simplify.cc, doc/tl/tl.tex: Add some rules.
* tests/python/ltlsimple.py: Test them.
