As discussed with Jan Strejček.
* spot/twa/acc.cc, spot/twa/acc.hh (fin_unit_one_split): New function.
(fin_one_extract): Return the simplified acceptance condition as an
optimization.
* python/spot/impl.i: Bind this new function.
* tests/python/acc.py: New file, to test it.
* tests/Makefile.am: Add acc.py.
* spot/twaalgos/genem.cc, spot/twaalgos/genem.hh: Implement the
spot211 variant of the emptiness check.
* tests/python/genem.py: Test it.
* tests/python/acc_cond.ipynb: Adjust test for fin_one_extract.
