hierarchy: Make is_recurrence() public

* NEWS: Declare it.
* spot/tl/hierarchy.cc: Remove static keyword and anonymous namespace.
* spot/tl/hierarchy.hh: Declare function.

NEWS

@@ -51,6 +51,9 @@ New in spot 2.4.0.dev (not yet released)
     recognize more if the original language can not be expressed with
     a co-Büchi acceptance condition.
 
+  - The function spot::is_recurrence() is now public. It checks if a formula
+    has the reccurence property.
+
   Deprecation notices:
 
   (These functions still work but compilers emit warnings.)

spot/tl/hierarchy.cc

@@ -30,9 +30,8 @@
 
 namespace spot
 {
-  namespace
-  {
-    static bool is_recurrence(formula f, const twa_graph_ptr& aut)
+  bool
+  is_recurrence(formula f, const twa_graph_ptr& aut)
   {
     if (f.is_syntactic_recurrence() || is_universal(aut))
       return true;
@@ -41,7 +40,8 @@ namespace spot
     // BA will preserve determinism if possible.
     spot::postprocessor p;
     p.set_type(spot::postprocessor::Generic);
-      p.set_pref(spot::postprocessor::Deterministic);
+    p.set_pref(spot::postprocessor::Deterministic
+               | spot::postprocessor::SBAcc);
     p.set_level(spot::postprocessor::Low);
     auto dra = p.run(aut);
     if (dra->acc().is_generalized_buchi())
@@ -55,7 +55,6 @@ namespace spot
         return is_deterministic(ba);
       }
   }
-  }
 
   char mp_class(formula f)
   {

spot/tl/hierarchy.hh

@@ -20,9 +20,14 @@
 #pragma once
 
 #include <spot/tl/formula.hh>
+#include <spot/twa/fwd.hh>
 
 namespace spot
 {
+  /// \brief Return true if the formula has the recurrence property.
+  SPOT_API
+  bool is_recurrence(formula f, const twa_graph_ptr& aut);
+
   /// \brief Return the class of \a f in the temporal hierarchy of Manna
   /// and Pnueli (PODC'90).
   ///