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().

spot/twaalgos/postproc.cc

@@ -34,6 +34,7 @@
 #include <spot/twaalgos/sbacc.hh>
 #include <spot/twaalgos/sepsets.hh>
 #include <spot/twaalgos/determinize.hh>
+#include <spot/twaalgos/alternation.hh>
 
 namespace spot
 {
@@ -173,6 +174,14 @@ namespace spot
     if (type_ == BA || SBACC_)
       state_based_ = true;
 
+    if (a->is_alternating() &&
+        // We will probably have to revisit this condition later.
+        // Currently, the intent is that postprocessor should never
+        // return an alternating automaton, unless it is called with
+        // its laxest settings.
+        !(type_ == Generic && PREF_ == Any && level_ == Low))
+      a = remove_alternation(a);
+
     if (type_ != Generic && !a->acc().is_generalized_buchi())
       {
         a = to_generalized_buchi(a);

tests/core/alternating.test

@@ -133,3 +133,27 @@ properties: univ-branch trans-labels explicit-labels state-acc
 properties: very-weak
 EOF
 diff output expected
+
+
+cat >out.hoa <<EOF
+HOA: v1
+tool: "ltl3dra" "0.2.2"
+name: "VWAA for GFa"
+States: 3
+Start: 0
+acc-name: co-Buchi
+Acceptance: 1 Fin(0)
+AP: 1 "a"
+properties: trans-labels explicit-labels state-acc univ-branch very-weak
+--BODY--
+State: 0 "GF(a)"
+ [t] 1&0
+State: 1 "F(a)" {0}
+ [(0)] 2
+ [t] 1
+State: 2 "t"
+ [t] 2
+--END--
+EOF
+test 3 = `autfilt --stats=%s out.hoa`
+test 2 = `autfilt --tgba --stats=%s out.hoa`