postproc: preliminary support for alternating automata

* spot/twaalgos/postproc.cc: Call remove_alternation().
* tests/core/alternating.test: Additional test.

NEWS

@@ -5,6 +5,12 @@ New in spot 2.2.2.dev (Not yet released)
   * ltlcross supports translators that output weak alternating
     automata in the HOA format (not necessarily *very* weak).
 
+  * autfilt can read alternating automata.  This is still experimental
+    (see below). Some of the algorithms proposed by autfilt will
+    refuse to work because they have not yet been updated to work with
+    alternating automata, but in any case they should display a
+    diagnostic: if you see a crash, please report it.
+
   Library:
 
   * A twa is required to have at least one state, the initial state.
@@ -41,6 +47,13 @@ New in spot 2.2.2.dev (Not yet released)
       were existential edges.  As a consequence, acceptance
       information is not accurate.
 
+    - postprocessor will call remove_alternation() right away, so
+      it can be used as a way to transform alternating automata
+      into different sub-types of (generalized) Büchi automata.
+      Note that although prostprocessor optimize the resulting
+      automata, it still has no simplification algorithms that work
+      at the alternating automaton level.
+
   * twa objects have a new property, very-weak, that can be set or
     retrieved via twa::prop_very_weak(), and that can be tested by
     is_very_weak_automaton().