diff --git a/src/bin/dstar2tgba.cc b/src/bin/dstar2tgba.cc
index a706ceb77..5992d442d 100644
--- a/src/bin/dstar2tgba.cc
+++ b/src/bin/dstar2tgba.cc
@@ -307,8 +307,8 @@ namespace
 
       const xtime_t before = gethrxtime();
 
-      spot::tgba* nba = spot::dstar_to_tgba(daut);
-      const spot::tgba* aut = post.run(nba, 0);
+      auto nba = spot::dstar_to_tgba(daut);
+      auto aut = post.run(nba, 0);
 
       const xtime_t after = gethrxtime();
       const double prec = XTIME_PRECISION;
diff --git a/src/bin/ltlfilt.cc b/src/bin/ltlfilt.cc
index c37c7b0b2..0aa57a81f 100644
--- a/src/bin/ltlfilt.cc
+++ b/src/bin/ltlfilt.cc
@@ -548,8 +548,8 @@ namespace
       // other reasons.
       if (matched && obligation)
 	{
-	  spot::tgba* aut = ltl_to_tgba_fm(f, simpl.get_dict());
-	  spot::tgba* min = minimize_obligation(aut, f);
+	  auto aut = ltl_to_tgba_fm(f, simpl.get_dict());
+	  auto min = minimize_obligation(aut, f);
 	  assert(min);
 	  if (aut == min)
 	    {
diff --git a/src/dstarparse/dra2ba.cc b/src/dstarparse/dra2ba.cc
index 660a61947..a90356440 100644
--- a/src/dstarparse/dra2ba.cc
+++ b/src/dstarparse/dra2ba.cc
@@ -49,7 +49,7 @@ namespace spot
 
   // This function is defined in nra2nba.cc, and used only here.
   SPOT_LOCAL
-  tgba* nra_to_nba(const dstar_aut* nra, const tgba* aut);
+  tgba_digraph* nra_to_nba(const dstar_aut* nra, const tgba* aut);
 
   namespace
   {
@@ -320,7 +320,7 @@ namespace spot
   }
 
 
-  tgba* dra_to_ba(const dstar_aut* dra, bool* dba)
+  tgba_digraph* dra_to_ba(const dstar_aut* dra, bool* dba)
   {
     assert(dra->type == Rabin);
 
diff --git a/src/dstarparse/dstar2tgba.cc b/src/dstarparse/dstar2tgba.cc
index 321dbee21..cb323f127 100644
--- a/src/dstarparse/dstar2tgba.cc
+++ b/src/dstarparse/dstar2tgba.cc
@@ -21,7 +21,7 @@
 
 namespace spot
 {
-  tgba*
+  tgba_digraph*
   dstar_to_tgba(const dstar_aut* daut)
   {
     switch (daut->type)
diff --git a/src/dstarparse/nra2nba.cc b/src/dstarparse/nra2nba.cc
index 3cb98294d..3df34ad28 100644
--- a/src/dstarparse/nra2nba.cc
+++ b/src/dstarparse/nra2nba.cc
@@ -122,7 +122,7 @@ namespace spot
   // In dra_to_dba() we call this function with a second argument
   // that is a masked version of nra->aut.
   SPOT_LOCAL
-  tgba* nra_to_nba(const dstar_aut* nra, const tgba* aut)
+  tgba_digraph* nra_to_nba(const dstar_aut* nra, const tgba* aut)
   {
     assert(nra->type == Rabin);
     nra_to_nba_worker w(nra, aut);
@@ -134,7 +134,7 @@ namespace spot
   }
 
   SPOT_API
-  tgba* nra_to_nba(const dstar_aut* nra)
+  tgba_digraph* nra_to_nba(const dstar_aut* nra)
   {
     return nra_to_nba(nra, nra->aut);
   }
diff --git a/src/dstarparse/nsa2tgba.cc b/src/dstarparse/nsa2tgba.cc
index 03df2a1f8..332e17046 100644
--- a/src/dstarparse/nsa2tgba.cc
+++ b/src/dstarparse/nsa2tgba.cc
@@ -99,7 +99,7 @@ namespace spot
   }
 
   SPOT_API
-  tgba* nsa_to_tgba(const dstar_aut* nsa)
+  tgba_digraph* nsa_to_tgba(const dstar_aut* nsa)
   {
     assert(nsa->type == Streett);
     tgba_digraph* a = nsa->aut;
diff --git a/src/dstarparse/public.hh b/src/dstarparse/public.hh
index 749217a88..ce03eecc2 100644
--- a/src/dstarparse/public.hh
+++ b/src/dstarparse/public.hh
@@ -106,14 +106,14 @@ namespace spot
 
   /// \brief Convert a non-deterministic Rabin automaton into a
   /// non-deterministic Büchi automaton.
-  SPOT_API tgba*
+  SPOT_API tgba_digraph*
   nra_to_nba(const dstar_aut* nra);
 
   /// \brief Convert a non-deterministic Rabin automaton into a
   /// non-deterministic Büchi automaton.
   ///
   /// This version simply ignores all states in \a ignore.
-  SPOT_API tgba*
+  SPOT_API tgba_digraph*
   nra_to_nba(const dstar_aut* nra, const state_set* ignore);
 
   /// \brief Convert a deterministic Rabin automaton into a
@@ -132,18 +132,18 @@ namespace spot
   /// If the optional \a dba_output argument is non-null, the
   /// pointed Boolean will be updated to indicate whether the
   /// returned Büchi automaton is deterministic.
-  SPOT_API tgba*
+  SPOT_API tgba_digraph*
   dra_to_ba(const dstar_aut* dra, bool* dba_output = 0);
 
   /// \brief Convert a non-deterministic Streett automaton into a
   /// non-deterministic tgba.
-  SPOT_API tgba*
+  SPOT_API tgba_digraph*
   nsa_to_tgba(const dstar_aut* nra);
 
   /// \brief Convert a Rabin or Streett automaton into a TGBA.
   ///
   /// This function calls dra_to_ba() or nsa_to_tgba().
-  SPOT_API tgba*
+  SPOT_API tgba_digraph*
   dstar_to_tgba(const dstar_aut* dstar);
 
 
diff --git a/src/tgba/tgbagraph.hh b/src/tgba/tgbagraph.hh
index 2c8937517..986d0a497 100644
--- a/src/tgba/tgbagraph.hh
+++ b/src/tgba/tgbagraph.hh
@@ -411,6 +411,48 @@ namespace spot
 	}
       g_.defrag();
     }
+
+
+  protected:
+    unsigned bprops_ = 0;
+
+  public:
+    enum bprop {
+      StateBasedAcc = 1,
+      SingleAccSet = 2,
+      SBA = StateBasedAcc | SingleAccSet,
+    };
+
+    bool get_bprop(bprop p) const
+    {
+      return (bprops_ & p) == p;
+    }
+
+    void set_bprop(bprop p)
+    {
+      bprops_ |= p;
+    }
+
+    void clear_bprop(bprop p)
+    {
+      bprops_ &= ~p;
+    }
+
+    bool state_is_accepting(unsigned s) const
+    {
+      assert(get_bprop(StateBasedAcc));
+      for (auto& t: g_.out(s))
+	// Stop at the first transition, since the remaining should be
+	// labeled identically.
+	return t.acc == all_acceptance_conditions_;
+      return false;
+    }
+
+    bool state_is_accepting(const state* s) const
+    {
+      return state_is_accepting(state_number(s));
+    }
+
   };
 
 }
diff --git a/src/tgba/tgbasafracomplement.cc b/src/tgba/tgbasafracomplement.cc
index ab0fd1edd..d72f3d282 100644
--- a/src/tgba/tgbasafracomplement.cc
+++ b/src/tgba/tgbasafracomplement.cc
@@ -34,7 +34,6 @@
 #include "ltlast/constant.hh"
 #include "tgbaalgos/dotty.hh"
 #include "tgba/tgbasafracomplement.hh"
-#include "tgba/sba.hh"
 #include "tgbaalgos/degen.hh"
 
 namespace spot
@@ -54,7 +53,7 @@ namespace spot
     /// \brief Automaton with Safra's tree as states.
     struct safra_tree_automaton
     {
-      safra_tree_automaton(const tgba* sba);
+      safra_tree_automaton(const tgba_digraph* sba);
       ~safra_tree_automaton();
       typedef std::map<bdd, const safra_tree*, bdd_less_than> transition_list;
       typedef
@@ -67,14 +66,14 @@ namespace spot
       int get_nb_acceptance_pairs() const;
       safra_tree* get_initial_state() const;
       void set_initial_state(safra_tree* s);
-      const tgba* get_sba(void) const
+      const tgba_digraph* get_sba(void) const
       {
 	return a_;
       }
     private:
       mutable int max_nb_pairs_;
       safra_tree* initial_state;
-      const tgba* a_;
+      const tgba_digraph* a_;
     };
 
     /// \brief A Safra tree, used as state during the determinization
@@ -112,7 +111,7 @@ namespace spot
       int max_name() const;
 
       // Operations to get successors of a tree.
-      safra_tree* branch_accepting(const sba& a);
+      safra_tree* branch_accepting(const tgba_digraph& a);
       safra_tree* succ_create(const bdd& condition,
                               cache_t& cache_transition);
       safra_tree* normalize_siblings();
@@ -316,7 +315,7 @@ namespace spot
     /// is inserted with the set of all accepting states of \c nodes
     /// as label and an unused name.
     safra_tree*
-    safra_tree::branch_accepting(const sba& a)
+    safra_tree::branch_accepting(const tgba_digraph& a)
     {
       for (auto c: children)
         c->branch_accepting(a);
@@ -574,7 +573,8 @@ namespace spot
     private:
       typedef std::set<int> atomic_list_t;
       typedef std::set<bdd, bdd_less_than> conjunction_list_t;
-      static void retrieve_atomics(const safra_tree* node, sba* sba_aut,
+      static void retrieve_atomics(const safra_tree* node,
+				   tgba_digraph* sba_aut,
                                    safra_tree::cache_t& cache,
                                    atomic_list_t& atomic_list);
       static void set_atomic_list(atomic_list_t& list, bdd condition);
@@ -587,7 +587,7 @@ namespace spot
     safra_determinisation::create_safra_automaton(const tgba* a)
     {
       // initialization.
-      sba* sba_aut = degeneralize(a);
+      auto sba_aut = degeneralize(a);
 
       safra_tree_automaton* st = new safra_tree_automaton(sba_aut);
 
@@ -664,7 +664,7 @@ namespace spot
     /// of the states in the label of the node.
     void
     safra_determinisation::retrieve_atomics(const safra_tree* node,
-                                            sba* sba_aut,
+                                            tgba_digraph* sba_aut,
                                             safra_tree::cache_t& cache,
                                             atomic_list_t& atomic_list)
     {
@@ -1031,7 +1031,7 @@ namespace spot
   // safra_tree_automaton
   ////////////////////////
 
-  safra_tree_automaton::safra_tree_automaton(const tgba* a)
+  safra_tree_automaton::safra_tree_automaton(const tgba_digraph* a)
     : max_nb_pairs_(-1), initial_state(0), a_(a)
   {
     a->get_dict()->register_all_variables_of(a, this);
diff --git a/src/tgbaalgos/compsusp.cc b/src/tgbaalgos/compsusp.cc
index 770e67ab6..b74806380 100644
--- a/src/tgbaalgos/compsusp.cc
+++ b/src/tgbaalgos/compsusp.cc
@@ -349,7 +349,7 @@ namespace spot
   }
 
 
-  tgba*
+  tgba_digraph*
   compsusp(const ltl::formula* f, bdd_dict* dict,
 	   bool no_wdba, bool no_simulation,
 	   bool early_susp, bool no_susp_product, bool wdba_smaller,
@@ -371,18 +371,11 @@ namespace spot
 
     if (!no_wdba)
       {
-	tgba* min = minimize_obligation(res, g, 0, wdba_smaller);
+	tgba_digraph* min = minimize_obligation(res, g, 0, wdba_smaller);
 	if (min != res)
 	  {
 	    delete res;
-	    // FIXME: minimize_obligation does not yet return a
-	    // tgba_digraph, so we convert the result using dupexp.
-	    // Once minimize_obligation is fixed, we should remove the
-	    // call to dupexp.
-	    assert(dynamic_cast<tgba_digraph*>(min) == nullptr);
-	    res = tgba_dupexp_dfs(min);
-	    delete min;
-	    //res = min;
+	    res = min;
 	    no_simulation = true;
 	  }
       }
diff --git a/src/tgbaalgos/compsusp.hh b/src/tgbaalgos/compsusp.hh
index 10fe47afb..d8c3fb3c6 100644
--- a/src/tgbaalgos/compsusp.hh
+++ b/src/tgbaalgos/compsusp.hh
@@ -21,6 +21,7 @@
 # define SPOT_TGBAALGOS_COMPSUSP_HH
 
 #include "ltlast/formula.hh"
+#include "tgba/tgbagraph.hh"
 
 namespace spot
 {
@@ -49,7 +50,7 @@ namespace spot
   /// This interface is subject to change, and clients aiming for
   /// long-term stability should better use the services of the
   /// spot::translator class instead.
-  SPOT_API tgba*
+  SPOT_API tgba_digraph*
   compsusp(const ltl::formula* f, bdd_dict* dict,
 	   bool no_wdba = false, bool no_simulation = false,
 	   bool early_susp = false, bool no_susp_product = false,
diff --git a/src/tgbaalgos/degen.cc b/src/tgbaalgos/degen.cc
index deb928cd6..2ea97a431 100644
--- a/src/tgbaalgos/degen.cc
+++ b/src/tgbaalgos/degen.cc
@@ -19,7 +19,7 @@
 
 
 #include "degen.hh"
-#include "tgba/tgbaexplicit.hh"
+#include "tgba/tgbagraph.hh"
 #include "misc/hash.hh"
 #include "misc/hashfunc.hh"
 #include "ltlast/constant.hh"
@@ -251,7 +251,7 @@ namespace spot
     };
   }
 
-  sba*
+  tgba_digraph*
   degeneralize(const tgba* a, bool use_z_lvl, bool use_cust_acc_orders,
                int use_lvl_cache, bool skip_levels)
   {
@@ -259,8 +259,9 @@ namespace spot
 
     bdd_dict* dict = a->get_dict();
 
-    // The result (degeneralized) automaton uses numbered states.
-    sba_explicit_number* res = new sba_explicit_number(dict);
+    // The result automaton is an SBA.
+    auto res = new tgba_digraph(dict);
+    res->set_bprop(tgba_digraph::SBA);
 
     // We use the same BDD variables as the input, except for the
     // acceptance.
@@ -314,7 +315,7 @@ namespace spot
     // (dest*2+acc) where dest is the destination state number, and
     // acc is 1 iff the transition is accepting.  The source
     // is always that of the current iteration.
-    typedef std::map<int, state_explicit_number::transition*> tr_cache_t;
+    typedef std::map<int, unsigned> tr_cache_t;
     tr_cache_t tr_cache;
 
     // State level cache
@@ -355,16 +356,7 @@ namespace spot
 	    }
       }
 
-#ifdef DEGEN_DEBUG
-    std::map<const state*, int>names;
-    names[s.first] = 1;
-
-    ds2num[s] =
-      10000 * names[s.first] + 100 * s.second + m.scc_of_state(s.first);
-#else
-    ds2num[s] = 0;
-#endif
-
+    ds2num[s] = res->new_state();
     todo.push_back(s);
 
     // If use_lvl_cache is on insert initial state to level cache
@@ -557,11 +549,7 @@ namespace spot
               }
             else
               {
-#ifdef DEGEN_DEBUG
-                dest = 10000 * names[d.first] + 100 * d.second + scc;
-#else
-                dest = ds2num.size();
-#endif
+                dest = res->new_state();
                 ds2num[d] = dest;
                 todo.push_back(d);
                 // Insert new state to cache
@@ -582,24 +570,24 @@ namespace spot
 		  }
               }
 
-            state_explicit_number::transition*& t =
-              tr_cache[dest * 2 + is_acc];
+            unsigned& t = tr_cache[dest * 2 + is_acc];
 
             if (t == 0)
               {
-                // Actually create the transition.
-                t = res->create_transition(src, dest);
-                t->condition = i->current_condition();
-                // If the source state is accepting, we have to put
-                // degen_acc on all outgoing transitions.  (We are still
-                // building a TGBA; we only assure that it can be used as
-                // an SBA.)
+                // Actually create the transition.  If the source
+                // state is accepting, we have to put degen_acc on all
+                // outgoing transitions.  (We are still building a
+                // TGBA; we only assure that it can be used as an
+                // SBA.)
+		bdd acc = bddfalse;
 		if (is_acc)
-		  t->acceptance_conditions = degen_acc;
+		  acc = degen_acc;
+                t = res->new_transition(src, dest,
+					i->current_condition(), acc);
               }
             else
               {
-                t->condition |= i->current_condition();
+                res->trans_data(t).cond |= i->current_condition();
               }
           }
         tr_cache.clear();
diff --git a/src/tgbaalgos/degen.hh b/src/tgbaalgos/degen.hh
index c67528bed..9d9d39d6b 100644
--- a/src/tgbaalgos/degen.hh
+++ b/src/tgbaalgos/degen.hh
@@ -20,13 +20,10 @@
 #ifndef SPOT_TGBAALGOS_DEGEN_HH
 # define SPOT_TGBAALGOS_DEGEN_HH
 
-# include "misc/common.hh"
+# include "tgba/tgbagraph.hh"
 
 namespace spot
 {
-  class sba;
-  class tgba;
-
   /// \ingroup tgba_misc
   /// \brief Degeneralize a spot::tgba into an equivalent sba with
   /// only one acceptance condition.
@@ -52,7 +49,7 @@ namespace spot
   /// \a a to be computed prior to its actual degeneralization.
   ///
   /// \see tgba_sba_proxy, tgba_tba_proxy
-  SPOT_API sba*
+  SPOT_API tgba_digraph*
   degeneralize(const tgba* a, bool use_z_lvl = true,
 	       bool use_cust_acc_orders = false,
 	       int use_lvl_cache = 1,
diff --git a/src/tgbaalgos/dotty.cc b/src/tgbaalgos/dotty.cc
index 2cd13d5bc..8bb4f3b00 100644
--- a/src/tgbaalgos/dotty.cc
+++ b/src/tgbaalgos/dotty.cc
@@ -28,7 +28,7 @@
 #include "tgba/bddprint.hh"
 #include "reachiter.hh"
 #include "misc/escape.hh"
-#include "tgba/tgbatba.hh"
+#include "tgba/tgbagraph.hh"
 #include "tgba/formula2bdd.hh"
 
 namespace spot
@@ -130,6 +130,9 @@ namespace spot
   {
     if (!dd)
       dd = dotty_decorator::instance();
+    if (const tgba_digraph* gd = dynamic_cast<const tgba_digraph*>(g))
+      assume_sba |= gd->get_bprop(tgba_digraph::StateBasedAcc);
+
     dotty_bfs d(os, g, assume_sba, dd);
     d.run();
     return os;
diff --git a/src/tgbaalgos/minimize.cc b/src/tgbaalgos/minimize.cc
index fe85ae906..f29a57cb3 100644
--- a/src/tgbaalgos/minimize.cc
+++ b/src/tgbaalgos/minimize.cc
@@ -48,7 +48,6 @@
 #include "tgbaalgos/bfssteps.hh"
 #include "tgbaalgos/isdet.hh"
 #include "tgbaalgos/dtgbacomp.hh"
-#include "priv/countstates.hh"
 
 namespace spot
 {
@@ -115,30 +114,43 @@ namespace spot
 
   // From the base automaton and the list of sets, build the minimal
   // resulting automaton
-  sba_explicit_number* build_result(const tgba* a,
-                                     std::list<hash_set*>& sets,
-                                     hash_set* final)
+  tgba_digraph* build_result(const tgba* a,
+			     std::list<hash_set*>& sets,
+			     hash_set* final)
   {
+    auto dict = a->get_dict();
+    auto res = new tgba_digraph(dict);
+    dict->register_all_variables_of(a, res);
+    dict->unregister_all_typed_variables(bdd_dict::acc, res);
+    res->set_bprop(tgba_digraph::StateBasedAcc);
+
     // For each set, create a state in the resulting automaton.
     // For a state s, state_num[s] is the number of the state in the minimal
     // automaton.
     hash_map state_num;
     std::list<hash_set*>::iterator sit;
-    unsigned num = 0;
     for (sit = sets.begin(); sit != sets.end(); ++sit)
       {
 	hash_set::iterator hit;
 	hash_set* h = *sit;
+	unsigned num = res->new_state();
 	for (hit = h->begin(); hit != h->end(); ++hit)
 	  state_num[*hit] = num;
-	++num;
       }
-    typedef state_explicit_number::transition trs;
-    sba_explicit_number* res = new sba_explicit_number(a->get_dict());
+
     // For each transition in the initial automaton, add the corresponding
     // transition in res.
+    bdd allacc = bddfalse;
     if (!final->empty())
-      res->declare_acceptance_condition(ltl::constant::true_instance());
+      {
+	res->set_bprop(tgba_digraph::SingleAccSet);
+	int accvar =
+	  dict->register_acceptance_variable(ltl::constant::true_instance(),
+					     res);
+	allacc = bdd_ithvar(accvar);
+	res->set_acceptance_conditions(allacc);
+      }
+
     for (sit = sets.begin(); sit != sets.end(); ++sit)
       {
 	hash_set::iterator hit;
@@ -157,17 +169,21 @@ namespace spot
 	    dst->destroy();
 	    if (i == state_num.end()) // Ignore useless destinations.
 	      continue;
-	    trs* t = res->create_transition(src_num, i->second);
-	    res->add_conditions(t, succit->current_condition());
+	    bdd acc = bddfalse;
 	    if (accepting)
-	      res->add_acceptance_condition(t, ltl::constant::true_instance());
+	      acc = allacc;
+	    res->new_transition(src_num, i->second,
+				succit->current_condition(), acc);
 	  }
       }
     res->merge_transitions();
-    const state* init_state = a->get_init_state();
-    unsigned init_num = state_num[init_state];
-    init_state->destroy();
-    res->set_init_state(init_num);
+    if (res->num_states() > 0)
+      {
+	const state* init_state = a->get_init_state();
+	unsigned init_num = state_num[init_state];
+	init_state->destroy();
+	res->set_init_state(init_num);
+      }
     return res;
   }
 
@@ -212,6 +228,7 @@ namespace spot
     wdba_scc_is_accepting(const tgba_digraph* det_a, unsigned scc_n,
 			  const tgba* orig_a, scc_map& sm, power_map& pm)
     {
+
       // Get some state from the SCC #n.
       const state* start = sm.one_state_of(scc_n)->clone();
 
@@ -272,8 +289,8 @@ namespace spot
 
   }
 
-  sba_explicit_number* minimize_dfa(const tgba_digraph* det_a,
-				    hash_set* final, hash_set* non_final)
+  tgba_digraph* minimize_dfa(const tgba_digraph* det_a,
+			     hash_set* final, hash_set* non_final)
   {
     typedef std::list<hash_set*> partition_t;
     partition_t cur_run;
@@ -460,7 +477,7 @@ namespace spot
 #endif
 
     // Build the result.
-    sba_explicit_number* res = build_result(det_a, done, final_copy);
+    auto* res = build_result(det_a, done, final_copy);
 
     // Free all the allocated memory.
     delete final_copy;
@@ -479,7 +496,7 @@ namespace spot
   }
 
 
-  sba_explicit_number* minimize_monitor(const tgba* a)
+  tgba_digraph* minimize_monitor(const tgba* a)
   {
     hash_set* final = new hash_set;
     hash_set* non_final = new hash_set;
@@ -497,7 +514,7 @@ namespace spot
     return minimize_dfa(det_a, final, non_final);
   }
 
-  sba_explicit_number* minimize_wdba(const tgba* a)
+  tgba_digraph* minimize_wdba(const tgba* a)
   {
     hash_set* final = new hash_set;
     hash_set* non_final = new hash_set;
@@ -596,21 +613,21 @@ namespace spot
     return minimize_dfa(det_a, final, non_final);
   }
 
-  tgba*
-  minimize_obligation(const tgba* aut_f,
+  tgba_digraph*
+  minimize_obligation(const tgba_digraph* aut_f,
 		      const ltl::formula* f, const tgba* aut_neg_f,
 		      bool reject_bigger)
   {
-    sba_explicit_number* min_aut_f = minimize_wdba(aut_f);
+    auto min_aut_f = minimize_wdba(aut_f);
 
     if (reject_bigger)
       {
 	// Abort if min_aut_f has more states than aut_f.
-	unsigned orig_states = count_states(aut_f);
+	unsigned orig_states = aut_f->num_states();
 	if (orig_states < min_aut_f->num_states())
 	  {
 	    delete min_aut_f;
-	    return const_cast<tgba*>(aut_f);
+	    return const_cast<tgba_digraph*>(aut_f);
 	  }
       }
 
@@ -653,7 +670,7 @@ namespace spot
 	  {
 	    // Otherwise, we cannot check if the minimization is safe.
 	    delete min_aut_f;
-	    return 0;
+	    return nullptr;
 	  }
       }
 
@@ -705,6 +722,6 @@ namespace spot
     if (ok)
       return min_aut_f;
     delete min_aut_f;
-    return const_cast<tgba*>(aut_f);
+    return const_cast<tgba_digraph*>(aut_f);
   }
 }
diff --git a/src/tgbaalgos/minimize.hh b/src/tgbaalgos/minimize.hh
index fb91fb296..f8f9c23d5 100644
--- a/src/tgbaalgos/minimize.hh
+++ b/src/tgbaalgos/minimize.hh
@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2011, 2012, 2013 Laboratoire de Recherche
-// et Développement de l'Epita (LRDE).
+// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014 Laboratoire de
+// Recherche et Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -20,7 +20,7 @@
 #ifndef SPOT_TGBAALGOS_MINIMIZE_HH
 # define SPOT_TGBAALGOS_MINIMIZE_HH
 
-# include "tgba/tgbaexplicit.hh"
+# include "tgba/tgbagraph.hh"
 # include "ltlast/formula.hh"
 
 namespace spot
@@ -57,7 +57,7 @@ namespace spot
   /// \param a the automaton to convert into a minimal deterministic monitor
   /// \pre Dead SCCs should have been removed from \a a before
   ///      calling this function.
-  SPOT_API sba_explicit_number* minimize_monitor(const tgba* a);
+  SPOT_API tgba_digraph* minimize_monitor(const tgba* a);
 
   /// \brief Minimize a Büchi automaton in the WDBA class.
   ///
@@ -93,7 +93,7 @@ namespace spot
         month     = oct
       }
       \endverbatim */
-  SPOT_API sba_explicit_number* minimize_wdba(const tgba* a);
+  SPOT_API tgba_digraph* minimize_wdba(const tgba* a);
 
   /// \brief Minimize an automaton if it represents an obligation property.
   ///
@@ -150,10 +150,10 @@ namespace spot
   /// determinization step during minimize_wdba().)  Note that
   /// checking the size of the minimized WDBA occurs before ensuring
   /// that the minimized WDBA is correct.
-  SPOT_API tgba* minimize_obligation(const tgba* aut_f,
-				     const ltl::formula* f = 0,
-				     const tgba* aut_neg_f = 0,
-				     bool reject_bigger = false);
+  SPOT_API tgba_digraph* minimize_obligation(const tgba_digraph* aut_f,
+					     const ltl::formula* f = 0,
+					     const tgba* aut_neg_f = 0,
+					     bool reject_bigger = false);
 
   /// @}
 }
diff --git a/src/tgbaalgos/postproc.cc b/src/tgbaalgos/postproc.cc
index e7d6e6d3c..d8fe0a842 100644
--- a/src/tgbaalgos/postproc.cc
+++ b/src/tgbaalgos/postproc.cc
@@ -75,7 +75,7 @@ namespace spot
       }
   }
 
-  const tgba* postprocessor::do_simul(const tgba* a, int opt)
+  const tgba_digraph* postprocessor::do_simul(const tgba_digraph* a, int opt)
   {
     switch (opt)
       {
@@ -95,7 +95,8 @@ namespace spot
       }
   }
 
-  const tgba* postprocessor::do_ba_simul(const tgba* a, int opt)
+  const tgba_digraph* postprocessor::do_ba_simul(const tgba_digraph* a,
+						 int opt)
   {
     switch (opt)
       {
@@ -112,18 +113,16 @@ namespace spot
   }
 
 
-  const tgba* postprocessor::do_degen(const tgba* a)
+  const tgba_digraph* postprocessor::do_degen(const tgba_digraph* a)
   {
-    const tgba* d = degeneralize(a,
-				 degen_reset_,
-				 degen_order_,
-				 degen_cache_,
-				 degen_lskip_);
+    auto d = degeneralize(a,
+			  degen_reset_, degen_order_,
+			  degen_cache_, degen_lskip_);
     delete a;
     if (ba_simul_ <= 0)
       return d;
 
-    const tgba* s = do_ba_simul(d, ba_simul_);
+    auto s = do_ba_simul(d, ba_simul_);
     if (s != d)
       delete d;
 
@@ -133,7 +132,8 @@ namespace spot
 #define PREF_ (pref_ & (Small | Deterministic))
 #define COMP_ (pref_ & Complete)
 
-  const tgba* postprocessor::run(const tgba* a, const ltl::formula* f)
+  const tgba_digraph*
+  postprocessor::run(const tgba_digraph* a, const ltl::formula* f)
   {
     if (type_ == TGBA && PREF_ == Any && level_ == Low)
       return a;
@@ -154,13 +154,13 @@ namespace spot
       {
 	// Do not bother about acceptance conditions, they will be
 	// ignored.
-	const tgba* s = scc_filter_states(a);
+	auto s = scc_filter_states(a);
 	delete a;
 	a = s;
       }
     else if (scc_filter_ > 0)
       {
-	const tgba* s = scc_filter(a, scc_filter_ > 1);
+	auto s = scc_filter(a, scc_filter_ > 1);
 	delete a;
 	a = s;
       }
@@ -169,20 +169,18 @@ namespace spot
       {
 	if (PREF_ == Deterministic)
 	  {
-	    const tgba* m = minimize_monitor(a);
+	    auto m = minimize_monitor(a);
 	    delete a;
 	    return m;
 	  }
 	else
 	  {
-	    const tgba* m = strip_acceptance(a);
-	    delete a;
-	    a = m;
+	    strip_acceptance_here(const_cast<tgba_digraph*>(a));
 	  }
 	if (PREF_ == Any)
 	  return a;
 
-	const tgba* sim = do_simul(a, simul_);
+	auto sim = do_simul(a, simul_);
 	if (a == sim)
 	  // simulation was disabled.
 	  return a;
@@ -193,9 +191,9 @@ namespace spot
 	  }
 	// For Small,High we return the smallest between the output of
 	// the simulation, and that of the deterministic minimization.
-	const tgba* m = minimize_monitor(a);
+	auto m = minimize_monitor(a);
 	delete a;
-	if (count_states(m) > count_states(sim))
+	if (m->num_states() > sim->num_states())
 	  {
 	    delete m;
 	  }
@@ -206,7 +204,7 @@ namespace spot
 	  }
 	if (COMP_ == Complete)
 	  {
-	    const tgba* s = tgba_complete(sim);
+	    auto s = tgba_complete(sim);
 	    delete sim;
 	    sim = s;
 	  }
@@ -222,8 +220,8 @@ namespace spot
 
     bool dba_is_wdba = false;
     bool dba_is_minimal = false;
-    const tgba* dba = 0;
-    const tgba* sim = 0;
+    const tgba_digraph* dba = 0;
+    const tgba_digraph* sim = 0;
 
     // (Small,Low) is the only configuration where we do not run
     // WDBA-minimization.
@@ -231,7 +229,7 @@ namespace spot
       {
 	bool reject_bigger = (PREF_ == Small) && (level_ == Medium);
 	dba = minimize_obligation(a, f, 0, reject_bigger);
-	if (dba == a)	// Minimization failed.
+	if (dba == a || dba == 0) // Minimization failed.
 	  dba = 0;
 	else
 	  dba_is_minimal = dba_is_wdba = true;
@@ -243,7 +241,6 @@ namespace spot
     if (!dba || (level_ == High && PREF_ == Small))
       {
 	sim = do_simul(a, simul_);
-
 	if (sim != a)
 	  delete a;
 
@@ -363,8 +360,8 @@ namespace spot
 	    in = dba;
 	  }
 
-	const tgba* cmp = tgba_complete(in);
-	const tgba* res = 0;
+	const tgba_digraph* cmp = tgba_complete(in);
+	const tgba_digraph* res = 0;
 	if (target_acc == 1)
 	  {
 	    if (sat_states_ != -1)
@@ -408,14 +405,14 @@ namespace spot
 	&& !(dba_is_minimal && state_based_
 	     && dba->number_of_acceptance_conditions() == 1))
       {
-	const tgba* d = degeneralize(dba);
+	auto d = degeneralize(dba);
 	delete dba;
 	dba = d;
       }
 
     if (dba && sim)
       {
-	if (count_states(dba) > count_states(sim))
+	if (dba->num_states() > sim->num_states())
 	  {
 	    delete dba;
 	    dba = 0;
@@ -432,14 +429,14 @@ namespace spot
       {
 	if (dba && !dba_is_minimal) // WDBA is already clean.
 	  {
-	    const tgba* s = scc_filter(dba, true);
+	    auto s = scc_filter(dba, true);
 	    delete dba;
 	    assert(!sim);
 	    dba = s;
 	  }
 	else if (sim)
 	  {
-	    const tgba* s = scc_filter(sim, true);
+	    auto s = scc_filter(sim, true);
 	    delete sim;
 	    assert(!dba);
 	    sim = s;
@@ -450,7 +447,7 @@ namespace spot
 
     if (COMP_ == Complete)
       {
-	const tgba* s = tgba_complete(sim);
+	auto s = tgba_complete(sim);
 	delete sim;
 	sim = s;
       }
diff --git a/src/tgbaalgos/postproc.hh b/src/tgbaalgos/postproc.hh
index 52b812ec0..967de2df3 100644
--- a/src/tgbaalgos/postproc.hh
+++ b/src/tgbaalgos/postproc.hh
@@ -20,7 +20,7 @@
 #ifndef SPOT_TGBAALGOS_POSTPROC_HH
 # define SPOT_TGBAALGOS_POSTPROC_HH
 
-#include "tgba/tgba.hh"
+#include "tgba/tgbagraph.hh"
 
 namespace spot
 {
@@ -97,12 +97,13 @@ namespace spot
     }
 
     /// Return the optimized automaton and delete \a input_disown.
-    const tgba* run(const tgba* input_disown, const ltl::formula* f);
+    const tgba_digraph* run(const tgba_digraph* input_disown,
+			    const ltl::formula* f);
 
   protected:
-    const tgba* do_simul(const tgba* input, int opt);
-    const tgba* do_ba_simul(const tgba* input, int opt);
-    const tgba* do_degen(const tgba* input);
+    const tgba_digraph* do_simul(const tgba_digraph* input, int opt);
+    const tgba_digraph* do_ba_simul(const tgba_digraph* input, int opt);
+    const tgba_digraph* do_degen(const tgba_digraph* input);
 
     output_type type_;
     int pref_;
diff --git a/src/tgbaalgos/sccfilter.cc b/src/tgbaalgos/sccfilter.cc
index 52443c5cc..1c7b74ace 100644
--- a/src/tgbaalgos/sccfilter.cc
+++ b/src/tgbaalgos/sccfilter.cc
@@ -20,8 +20,8 @@
 #include "tgba/tgbaexplicit.hh"
 #include "sccfilter.hh"
 #include "reachiter.hh"
-#include "tgbaalgos/scc.hh"
-#include "tgbaalgos/sccinfo.hh"
+#include "scc.hh"
+#include "sccinfo.hh"
 
 namespace spot
 {
@@ -903,7 +903,8 @@ namespace spot
 
 	if (keep && acc != bddfalse)
 	  {
-	    unsigned u = this->si->scc_of(src);
+	    unsigned u = this->si->scc_of(dst);
+
 	    auto i = remap_[u].find(acc.id());
 	    if (i != remap_[u].end())
 	      acc = i->second;
@@ -920,6 +921,12 @@ namespace spot
     tgba_digraph* scc_filter_apply(const tgba_digraph* aut,
 				   scc_info* given_si, Args&&... args)
     {
+      bdd_dict* bd = aut->get_dict();
+      tgba_digraph* filtered = new tgba_digraph(bd);
+      unsigned in_n = aut->num_states(); // Number of input states.
+      if (in_n == 0)			 // Nothing to filter.
+	return filtered;
+
       // Compute scc_info if not supplied.
       scc_info* si = given_si;
       if (!si)
@@ -928,7 +935,6 @@ namespace spot
       F filter(si, std::forward<Args>(args)...);
 
       // Renumber all useful states.
-      unsigned in_n = aut->num_states(); // Number of input states.
       unsigned out_n = 0;		 // Number of output states.
       std::vector<unsigned> inout; // Associate old states to new ones.
       inout.reserve(in_n);
@@ -938,8 +944,6 @@ namespace spot
 	else
 	  inout.push_back(-1U);
 
-      bdd_dict* bd = aut->get_dict();
-      tgba_digraph* filtered = new tgba_digraph(bd);
       bd->register_all_variables_of(aut, filtered);
       {
 	bdd all = aut->all_acceptance_conditions();
@@ -989,14 +993,17 @@ namespace spot
   scc_filter(const tgba_digraph* aut, bool remove_all_useless,
 	     scc_info* given_si)
   {
+    tgba_digraph* res;
     if (remove_all_useless)
-      return scc_filter_apply<state_filter
-			      <acc_filter_all
-			       <acc_filter_simplify<>>>>(aut, given_si);
+      res = scc_filter_apply<state_filter
+			     <acc_filter_all
+			      <acc_filter_simplify<>>>>(aut, given_si);
     else
-      return scc_filter_apply<state_filter
-			      <acc_filter_some
-			       <acc_filter_simplify<>>>>(aut, given_si);
+      res = scc_filter_apply<state_filter
+			     <acc_filter_some
+			      <acc_filter_simplify<>>>>(aut, given_si);
+    res->merge_transitions();
+    return res;
   }
 
   tgba_digraph*
@@ -1004,22 +1011,25 @@ namespace spot
 		  bdd suspvars, bdd ignoredvars, bool early_susp,
 		  scc_info* given_si)
   {
+    tgba_digraph* res;
     if (remove_all_useless)
-      return scc_filter_apply<susp_filter
-			      <state_filter
-			       <acc_filter_all
-				<acc_filter_simplify<>>>>>(aut, given_si,
-							   suspvars,
-							   ignoredvars,
-							   early_susp);
+      res = scc_filter_apply<susp_filter
+			     <state_filter
+			      <acc_filter_all
+			       <acc_filter_simplify<>>>>>(aut, given_si,
+							  suspvars,
+							  ignoredvars,
+							  early_susp);
     else
-      return scc_filter_apply<susp_filter
-			      <state_filter
-			       <acc_filter_some
-				<acc_filter_simplify<>>>>>(aut, given_si,
-							   suspvars,
-							   ignoredvars,
-							   early_susp);
+      res = scc_filter_apply<susp_filter
+			     <state_filter
+			      <acc_filter_some
+			       <acc_filter_simplify<>>>>>(aut, given_si,
+							  suspvars,
+							  ignoredvars,
+							  early_susp);
+    res->merge_transitions();
+    return res;
   }
 
 }
diff --git a/src/tgbaalgos/sccinfo.cc b/src/tgbaalgos/sccinfo.cc
index 53b539d22..07d612bdb 100644
--- a/src/tgbaalgos/sccinfo.cc
+++ b/src/tgbaalgos/sccinfo.cc
@@ -76,13 +76,14 @@ namespace spot
 
 
     // Setup depth-first search from the initial state.
-    {
-      unsigned init = aut->get_init_state_number();
-      num_ = -1;
-      h_[init] = num_;
-      root_.emplace_front(num_, bddfalse, bddfalse);
-      todo_.emplace(init, aut->out(init).begin());
-    }
+    if (n > 0)
+      {
+	unsigned init = aut->get_init_state_number();
+	num_ = -1;
+	h_[init] = num_;
+	root_.emplace_front(num_, bddfalse, bddfalse);
+	todo_.emplace(init, aut->out(init).begin());
+      }
 
     while (!todo_.empty())
       {
@@ -237,7 +238,7 @@ namespace spot
     for (unsigned src = 0; src < n; ++src)
       {
 	unsigned src_scc = scc_of(src);
-	if (!is_accepting_scc(src_scc))
+	if (src_scc == -1U || !is_accepting_scc(src_scc))
 	  continue;
 	for (auto& t: aut_->out(src))
 	  {
diff --git a/src/tgbaalgos/stripacc.cc b/src/tgbaalgos/stripacc.cc
index da7e31c67..aced5b2cf 100644
--- a/src/tgbaalgos/stripacc.cc
+++ b/src/tgbaalgos/stripacc.cc
@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2012 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE).
+// Copyright (C) 2012, 2014 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -22,42 +22,13 @@
 
 namespace spot
 {
-  namespace
+  void strip_acceptance_here(tgba_digraph* a)
   {
-    class strip_iter: public tgba_reachable_iterator_depth_first
-    {
-    public:
-      strip_iter(const tgba* a)
-	: tgba_reachable_iterator_depth_first(a),
-	  out_(new sba_explicit_number(a->get_dict()))
-      {
-      }
-
-      sba_explicit_number*
-      result()
-      {
-	return out_;
-      }
-
-      void
-      process_link(const state*, int in,
-		   const state*, int out,
-		   const tgba_succ_iterator* si)
-      {
-	state_explicit_number::transition* t = out_->create_transition(in, out);
-	out_->add_conditions(t, si->current_condition());
-      }
-
-    private:
-      sba_explicit_number* out_;
-    };
-  }
-
-  sba_explicit_number*
-  strip_acceptance(const tgba* a)
-  {
-    strip_iter si(a);
-    si.run();
-    return si.result();
+    unsigned n = a->num_states();
+    for (unsigned s = 0; s < n; ++s)
+      for (auto& t: a->out(s))
+	t.acc = bddfalse;
+    a->set_acceptance_conditions(bddfalse);
+    a->get_dict()->unregister_all_typed_variables(bdd_dict::acc, a);
   }
 }
diff --git a/src/tgbaalgos/stripacc.hh b/src/tgbaalgos/stripacc.hh
index 2f92d2668..7ff3346e7 100644
--- a/src/tgbaalgos/stripacc.hh
+++ b/src/tgbaalgos/stripacc.hh
@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2012, 2013 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE).
+// Copyright (C) 2012, 2013, 2014 Laboratoire de Recherche et
+// Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -20,16 +20,16 @@
 #ifndef SPOT_TGBAALGOS_STRIPACC_HH
 # define SPOT_TGBAALGOS_STRIPACC_HH
 
-# include "tgba/tgbaexplicit.hh"
+# include "tgba/tgbagraph.hh"
 
 namespace spot
 {
   /// \ingroup tgba_misc
-  /// \brief Duplicate automaton \a a, removing all acceptance sets.
+  /// \brief Remove all acceptance sets from a tgba_digraph.
   ///
   /// This is equivalent to marking all states/transitions as accepting.
-  SPOT_API sba_explicit_number*
-  strip_acceptance(const tgba* a);
+  SPOT_API void
+  strip_acceptance_here(tgba_digraph* a);
 }
 
 #endif // SPOT_TGBAALGOS_STRIPACC_HH
diff --git a/src/tgbaalgos/translate.cc b/src/tgbaalgos/translate.cc
index c36ee5a75..fabfaa17f 100644
--- a/src/tgbaalgos/translate.cc
+++ b/src/tgbaalgos/translate.cc
@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2013 Laboratoire de Recherche et Développement
+// Copyright (C) 2013, 2014 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -61,7 +61,7 @@ namespace spot
     simpl_owned_ = simpl_ = new ltl::ltl_simplifier(options, dict);
   }
 
-  const tgba* translator::run(const ltl::formula** f)
+  const tgba_digraph* translator::run(const ltl::formula** f)
   {
     const ltl::formula* r = simpl_->simplify(*f);
     (*f)->destroy();
@@ -71,7 +71,7 @@ namespace spot
     // natural way (improving the degeneralization).
     simpl_->clear_as_bdd_cache();
 
-    const tgba* aut;
+    const tgba_digraph* aut;
     if (comp_susp_ > 0)
       {
 	int skel_wdba = skel_wdba_;
@@ -91,10 +91,10 @@ namespace spot
     return aut;
   }
 
-  const tgba* translator::run(const ltl::formula* f)
+  const tgba_digraph* translator::run(const ltl::formula* f)
   {
     f->clone();
-    const tgba* aut = run(&f);
+    auto aut = run(&f);
     f->destroy();
     return aut;
   }
diff --git a/src/tgbaalgos/translate.hh b/src/tgbaalgos/translate.hh
index ab626df6f..6d9dbd09d 100644
--- a/src/tgbaalgos/translate.hh
+++ b/src/tgbaalgos/translate.hh
@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2013 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE).
+// Copyright (C) 2013, 2014 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -103,14 +103,14 @@ namespace spot
     ///
     /// The formula \a f is simplified internally, but it is not
     /// not destroyed (this is the responsibility of the caller).
-    const tgba* run(const ltl::formula* f);
+    const tgba_digraph* run(const ltl::formula* f);
 
     /// \brief Convert \a f into an automaton, and update f.
     ///
     /// The formula <code>*f</code> is destroyed, and replaced
     /// by the simplified version, which should be destroyed by
     /// the caller.
-    const tgba* run(const ltl::formula** f);
+    const tgba_digraph* run(const ltl::formula** f);
 
   protected:
     void setup_opt(const option_map* opt);
diff --git a/src/tgbatest/ltl2tgba.cc b/src/tgbatest/ltl2tgba.cc
index f5b1144c1..d70ac48ca 100644
--- a/src/tgbatest/ltl2tgba.cc
+++ b/src/tgbatest/ltl2tgba.cc
@@ -1003,7 +1003,6 @@ main(int argc, char** argv)
 
       if (from_file)
 	{
-	  spot::tgba_explicit_string* e = 0;
 	  switch (readformat)
 	    {
 	    case ReadSpot:
@@ -1113,9 +1112,6 @@ main(int argc, char** argv)
 	      }
 	      break;
 	    }
-	  if (e)
-	    e->merge_transitions();
-
 	}
       else
 	{
@@ -1204,19 +1200,36 @@ main(int argc, char** argv)
       const spot::tgba* aut_scc = 0;
       if (scc_filter)
 	{
+	  auto aa = dynamic_cast<const spot::tgba_digraph*>(a);
+	  bool freeit = false;
+	  if (!aa)
+	    {
+	      freeit = true;
+	      aa = tgba_dupexp_dfs(a);
+	    }
+	  assert(aa);
 	  tm.start("SCC-filter");
-	  aut_scc = a = spot::scc_filter(a, scc_filter_all);
+	  aut_scc = a = spot::scc_filter(aa, scc_filter_all);
 	  tm.stop("SCC-filter");
 	  assume_sba = false;
+	  if (freeit)
+	    delete aa;
 	}
 
       const spot::tgba* degeneralized = 0;
 
-      spot::tgba* minimized = 0;
+      spot::tgba_digraph* minimized = 0;
       if (opt_minimize)
 	{
 	  tm.start("obligation minimization");
-	  minimized = minimize_obligation(a, f, 0, reject_bigger);
+	  auto aa = dynamic_cast<const spot::tgba_digraph*>(a);
+	  bool freeit = false;
+	  if (!aa)
+	    {
+	      freeit = true;
+	      aa = tgba_dupexp_dfs(a);
+	    }
+	  minimized = minimize_obligation(aa, f, 0, reject_bigger);
 	  tm.stop("obligation minimization");
 
 	  if (minimized == 0)
@@ -1229,7 +1242,7 @@ main(int argc, char** argv)
 		  exit(2);
 		}
 	    }
-	  else if (minimized == a)
+	  else if (minimized == aa)
 	    {
 	      minimized = 0;
 	    }
@@ -1244,6 +1257,8 @@ main(int argc, char** argv)
               reduction_iterated_sim = false;
 	      assume_sba = true;
 	    }
+	  if (freeit)
+	    delete aa;
 	}
 
       if (reduction_dir_sim && !reduction_iterated_sim)
@@ -1302,8 +1317,10 @@ main(int argc, char** argv)
 
 	  if (scc_filter)
 	    {
+	      auto aa = down_cast<const spot::tgba_digraph*>(a);
+	      assert(aa);
 	      tm.start("SCC-filter on don't care");
-	      a = spot::scc_filter(a, true);
+	      a = spot::scc_filter(aa, true);
 	      delete temp_dont_care_sim;
 	      temp_dont_care_sim = a;
 	      tm.stop("SCC-filter on don't care");
@@ -1438,8 +1455,10 @@ main(int argc, char** argv)
 	    {
 	      tm.start("SCC-filter post-sim");
 	      delete aut_scc;
+	      auto aa = down_cast<const spot::tgba_digraph*>(a);
+	      assert(aa);
 	      // Do not filter_all for SBA
-	      aut_scc = a = spot::scc_filter(a, assume_sba ?
+	      aut_scc = a = spot::scc_filter(aa, assume_sba ?
 					     false : scc_filter_all);
 	      tm.stop("SCC-filter post-sim");
 	    }
@@ -1448,7 +1467,7 @@ main(int argc, char** argv)
       if (opt_monitor)
 	{
 	  tm.start("Monitor minimization");
-	  minimized = a = minimize_monitor(a);
+	  a = minimized = minimize_monitor(a);
 	  tm.stop("Monitor minimization");
 	  assume_sba = false; 	// All states are accepting, so double
 				// circles in the dot output are
@@ -1612,16 +1631,6 @@ main(int argc, char** argv)
 	  a = new spot::future_conditions_collector(a, true);
 	}
 
-      if (utf8_opt)
-	{
-	  if (spot::tgba_explicit_formula* tef =
-	      dynamic_cast<spot::tgba_explicit_formula*>(a))
-	    tef->enable_utf8();
-	  else if (spot::sba_explicit_formula* sef =
-		   dynamic_cast<spot::sba_explicit_formula*>(a))
-	    sef->enable_utf8();
-	}
-
       if (output != -1)
 	{
 	  tm.start("producing output");
diff --git a/wrap/python/spot.i b/wrap/python/spot.i
index 794b7d596..296945255 100644
--- a/wrap/python/spot.i
+++ b/wrap/python/spot.i
@@ -378,13 +378,22 @@ namespace std {
 %inline %{
 
 // A variant of minimize_obligation() that always return a new object.
-const spot::tgba*
+const spot::tgba_digraph*
 minimize_obligation_new(const spot::tgba* a, const spot::ltl::formula* f)
 {
-  const tgba* res = spot::minimize_obligation(a, f);
+  auto aa = dynamic_cast<const spot::tgba_digraph*>(a);
+  bool freeit = false;
+  if (!aa)
+    {
+      freeit = true;
+      aa = tgba_dupexp_dfs(a);
+    }
+  auto res = spot::minimize_obligation(aa, f);
   // Return 0 if the output is the same as the input, otherwise
   // it is hard for swig to know if the output is "new" or not.
-  if (res == a)
+  if (freeit)
+    delete aa;
+  if (res == aa)
     return 0;
   else
     return res;