Make it easier to convert acc-conditions to associated formulae.

This is motivated by the fact that sog-its used the low-level data
structures used by the bdd_dict to do such work, and broke because of
the recent changes in this area.

* src/tgba/bdddict.cc, src/tgba/bdddict.hh (oneacc_to_formula):
New method.
* src/tgbaalgos/save.cc: Use it.

src/tgba/bdddict.cc

@@ -159,7 +159,6 @@ namespace spot
     register_acceptance_variables(bdd_low(f), for_me);
   }
 
-
   int
   bdd_dict::register_clone_acc(int v, const void* for_me)
   {
@@ -178,6 +177,26 @@ namespace spot
     return res;
   }
 
+  const ltl::formula*
+  bdd_dict::oneacc_to_formula(int var) const
+  {
+    assert(unsigned(var) < bdd_map.size());
+    const bdd_info& i = bdd_map[var];
+    assert(i.type == acc);
+    return i.f;
+  }
+
+  const ltl::formula*
+  bdd_dict::oneacc_to_formula(bdd oneacc) const
+  {
+    assert(oneacc != bddfalse);
+    while (bdd_high(oneacc) == bddfalse)
+      {
+	oneacc = bdd_low(oneacc);
+	assert(oneacc != bddfalse);
+      }
+    return oneacc_to_formula(bdd_var(oneacc));
+  }
 
   int
   bdd_dict::register_anonymous_variables(int n, const void* for_me)

src/tgba/bdddict.hh

@@ -148,6 +148,27 @@ namespace spot
     /// automaton).
     void register_acceptance_variables(bdd f, const void* for_me);
 
+    /// \brief Convert one acceptance condition into the associated
+    /// formula.
+    ///
+    /// This version accepts a conjunction of Acc variables, in which
+    /// only one must be positive.  This positive variable will be
+    /// converted back into the associated formula.
+    ///
+    /// The returned formula is not cloned, and is valid until the BDD
+    /// variable used in \a oneacc are unregistered.
+    const ltl::formula* oneacc_to_formula(bdd oneacc) const;
+
+    /// \brief Convert one acceptance condition into the associated
+    /// formula.
+    ///
+    /// This version takes the number of a BDD variable that must has
+    /// been returned by a call to register_acceptance_variable().
+    ///
+    /// The returned formula is not cloned, and is valid until the BDD
+    /// variable \a var is unregistered.
+    const ltl::formula* oneacc_to_formula(int var) const;
+
     /// \brief Register anonymous BDD variables.
     ///
     /// Return (and maybe allocate) \a n consecutive BDD variables which