* spot/twaalgos/degen.cc, spot/twaalgos/degen.hh: Implement this.
Also throw a runtime error in case were todegen does not match any
subformula.
* tests/python/pdegen.py: Add tests.
On these 4 cases, added to pdegen.py, and supplied by Florian Renkin,
partial_degeneralize() is now at least as good as degeneralize_tba(),
and sometimes better. This is achieved as follows: (1) a
propagate_marks procedure is introduced to propagate marks as far as
possible on the automaton (e.g., common outgoing marks can be push
onto the incoming transitions and vice-versa), (2) the
degeneralization order is compute dynamically, and (3) whenever and
fully-accepting transition is taken in the original automaton, the
destination level is chosen to be the highest existing level.
* spot/twaalgos/degen.cc,
spot/twaalgos/degen.hh (propagate_marks_vector, propagate_marks_here):
New functions.
(partial_degeneralize): Improve, as described above.
* tests/python/pdegen.py: Add test cases.
* spot/twaalgos/degen.hh, spot/twaalgos/degen.cc: Implement it.
* tests/python/pdegen.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention the new function.
