introduce delay_branching_here

This is motivated by an example sent by Edmond Irani Liu,
that will be tested in next patch.

* spot/twaalgos/dbranch.cc, spot/twaalgos/dbranch.hh: New files.
* python/spot/impl.i, spot/twaalgos/Makefile.am: Add them.
* spot/twaalgos/translate.cc: Call delay_branching_here
unconditionally.
* spot/twa/twagraph.cc (defrag_states): Do not assume
that games are alternating.
* tests/core/genltl.test: Adjust expected numbers.
* tests/python/dbranch.py: New file.
* tests/Makefile.am: Add it.

NEWS

@@ -142,6 +142,15 @@ New in spot 2.10.6.dev  (not yet released)
     succesors, should be called before running simulation-based
     reductions.
 
+  - A new function delay_branching_here(aut) can be used to simplify
+    some non-deterministic branching.  If two transitions (q₁,ℓ,M,q₂)
+    and (q₁,ℓ,M,q₃) differ only by their destination state, and are
+    the only incoming transitions of their destination states, then q₂
+    and q₃ can be merged (taking the union of their outgoing
+    transitions).  This is cheap function is automatically called by
+    spot::translate() after translation of a formula to GBA, before
+    further simplification.
+
   - spot::parallel_policy is an object that can be passed to some
     algorithm to specify how many threads can be used if Spot has been
     compiled with --enable-pthread.  Currently, only