diff --git a/src/ltlast/binop.cc b/src/ltlast/binop.cc
index 3f08ccd61..c2866d4c3 100644
--- a/src/ltlast/binop.cc
+++ b/src/ltlast/binop.cc
@@ -106,9 +106,8 @@ namespace spot
 	  is.accepting_eword = false;
 	  break;
 	case EConcatMarked:
-	  is.not_marked = false;
-	  // fall through
 	case EConcat:
+	  is.not_marked = (op != EConcatMarked);
 	  is.ltl_formula = false;
 	  is.boolean = false;
 	  is.eltl_formula = false;
@@ -130,11 +129,11 @@ namespace spot
 	      is.syntactic_obligation = second->is_syntactic_guarantee();
 	      is.syntactic_recurrence = second->is_syntactic_guarantee();
 	    }
-
 	  assert(first->is_sere_formula());
 	  assert(second->is_psl_formula());
 	  break;
 	case UConcat:
+	  is.not_marked = true;
 	  is.ltl_formula = false;
 	  is.boolean = false;
 	  is.eltl_formula = false;
@@ -156,11 +155,11 @@ namespace spot
 	      is.syntactic_obligation = second->is_syntactic_safety();
 	      is.syntactic_persistence = second->is_syntactic_safety();
 	    }
-
 	  assert(first->is_sere_formula());
 	  assert(second->is_psl_formula());
 	  break;
 	case U:
+	  is.not_marked = true;
 	  // f U g is universal if g is eventual, or if f == 1.
 	  is.eventual = second->is_eventual();
 	  is.eventual |= (first == constant::true_instance());
@@ -181,6 +180,7 @@ namespace spot
 	  // is.syntactic_persistence = Persistence U Persistance
 	  break;
 	case W:
+	  is.not_marked = true;
 	  // f W g is universal if f and g are, or if g == 0.
 	  is.universal |= (second == constant::false_instance());
 	  is.boolean = false;
@@ -200,6 +200,7 @@ namespace spot
 
 	  break;
 	case R:
+	  is.not_marked = true;
 	  // g R f is universal if f is universal, or if g == 0.
 	  is.universal = second->is_universal();
 	  is.universal |= (first == constant::false_instance());
@@ -220,6 +221,7 @@ namespace spot
 
 	  break;
 	case M:
+	  is.not_marked = true;
 	  // g M f is eventual if both g and f are eventual, or if f == 1.
 	  is.eventual |= (second == constant::true_instance());
 	  is.boolean = false;
diff --git a/src/ltlast/unop.cc b/src/ltlast/unop.cc
index b17dc108e..05e3b1c58 100644
--- a/src/ltlast/unop.cc
+++ b/src/ltlast/unop.cc
@@ -39,6 +39,7 @@ namespace spot
       switch (op)
 	{
 	case Not:
+	  is.not_marked = true;
 	  is.eventual = child->is_universal();
 	  is.universal = child->is_eventual();
 	  is.in_nenoform = (child->kind() == AtomicProp);
@@ -53,6 +54,7 @@ namespace spot
 	  is.accepting_eword = false;
 	  break;
 	case X:
+	  is.not_marked = true;
 	  is.boolean = false;
 	  is.X_free = false;
 	  is.eltl_formula = false;
@@ -65,6 +67,7 @@ namespace spot
 	  is.accepting_eword = false;
 	  break;
 	case F:
+	  is.not_marked = true;
 	  is.boolean = false;
 	  is.eltl_formula = false;
 	  is.sere_formula = false;
@@ -79,6 +82,7 @@ namespace spot
 	  is.accepting_eword = false;
 	  break;
 	case G:
+	  is.not_marked = true;
 	  is.boolean = false;
 	  is.eltl_formula = false;
 	  is.sere_formula = false;
@@ -93,6 +97,7 @@ namespace spot
 	  is.accepting_eword = false;
 	  break;
 	case Finish:
+	  is.not_marked = true;
 	  is.boolean = false;
 	  is.ltl_formula = false;
 	  is.psl_formula = false;
@@ -120,6 +125,7 @@ namespace spot
 	  is.accepting_eword = false;
 	  break;
 	case Closure:
+	  is.not_marked = true;
 	  is.boolean = false;
 	  is.ltl_formula = false;
 	  is.eltl_formula = false;
diff --git a/src/ltlvisit/mark.cc b/src/ltlvisit/mark.cc
index 61c3a8dd3..05d834e0e 100644
--- a/src/ltlvisit/mark.cc
+++ b/src/ltlvisit/mark.cc
@@ -22,6 +22,8 @@
 #include "ltlast/allnodes.hh"
 #include <cassert>
 #include <algorithm>
+#include "ltlvisit/tostring.hh"
+#include "misc/casts.hh"
 
 namespace spot
 {
@@ -32,11 +34,11 @@ namespace spot
       class simplify_mark_visitor : public visitor
       {
 	formula* result_;
-	bool has_mark_;
+	mark_tools* tools_;
 
       public:
-	simplify_mark_visitor()
-	  : has_mark_(false)
+	simplify_mark_visitor(mark_tools* t)
+	  : tools_(t)
 	{
 	}
 
@@ -50,12 +52,6 @@ namespace spot
 	  return result_;
 	}
 
-	bool
-	has_mark()
-	{
-	  return has_mark_;
-	}
-
 	void
 	visit(atomic_prop* ao)
 	{
@@ -77,22 +73,7 @@ namespace spot
 	void
 	visit(unop* uo)
 	{
-	  switch (uo->op())
-	    {
-	    case unop::NegClosure:
-	      has_mark_ = true;
-	      /* fall through */
-	    case unop::Finish:
-	    case unop::Closure:
-	    case unop::Not:
-	    case unop::X:
-	    case unop::F:
-	    case unop::G:
-	      result_ = uo->clone();
-	      return;
-	    }
-	  /* Unreachable code. */
-	  assert(0);
+	  result_ = uo->clone();
 	}
 
 	void
@@ -150,7 +131,6 @@ namespace spot
 			  case binop::EConcatMarked:
 			    EMpairs.insert(std::make_pair(bo->first(),
 							  bo->second()));
-			    has_mark_ = true;
 			    break;
 			  }
 		      }
@@ -174,33 +154,13 @@ namespace spot
 	void
 	visit(binop* bo)
 	{
-	  switch (bo->op())
-	    {
-	    case binop::EConcatMarked:
-	      has_mark_ = true;
-	      /* fall through */
-	    case binop::Xor:
-	    case binop::Implies:
-	    case binop::Equiv:
-	    case binop::U:
-	    case binop::W:
-	    case binop::M:
-	    case binop::R:
-	    case binop::EConcat:
-	    case binop::UConcat:
-	      result_ = bo->clone();
-	      return;
-	    }
-	  /* Unreachable code. */
-	  assert(0);
+	  result_ = bo->clone();
 	}
 
 	formula*
-	recurse(const formula* f)
+	recurse(formula* f)
 	{
-	  formula* g = f->clone();
-	  has_mark_ |= simplify_mark(g);
-	  return g;
+	  return tools_->simplify_mark(f);
 	}
       };
 
@@ -208,8 +168,11 @@ namespace spot
       class mark_visitor : public visitor
       {
 	formula* result_;
+	mark_tools* tools_;
+
       public:
-	mark_visitor()
+	mark_visitor(mark_tools* t)
+	  : tools_(t)
 	{
 	}
 	~mark_visitor()
@@ -294,28 +257,74 @@ namespace spot
 	formula*
 	recurse(formula* f)
 	{
-	  return mark_concat_ops(f);
+	  return tools_->mark_concat_ops(f);
 	}
       };
 
     }
 
-    formula*
-    mark_concat_ops(const formula* f)
+    mark_tools::mark_tools()
+      : simpvisitor_(new simplify_mark_visitor(this)),
+	markvisitor_(new mark_visitor(this))
     {
-      mark_visitor v;
-      const_cast<formula*>(f)->accept(v);
-      return v.result();
     }
 
-    bool
-    simplify_mark(formula*& f)
+
+    mark_tools::~mark_tools()
     {
-      simplify_mark_visitor v;
-      const_cast<formula*>(f)->accept(v);
-      f->destroy();
-      f = v.result();
-      return v.has_mark();
+      delete simpvisitor_;
+      delete markvisitor_;
+      {
+	f2f_map::iterator i = simpmark_.begin();
+	f2f_map::iterator end = simpmark_.end();
+	while (i != end)
+	  {
+	    f2f_map::iterator old = i++;
+	    old->second->destroy();
+	    old->first->destroy();
+	  }
+      }
+      {
+	f2f_map::iterator i = markops_.begin();
+	f2f_map::iterator end = markops_.end();
+	while (i != end)
+	  {
+	    f2f_map::iterator old = i++;
+	    old->second->destroy();
+	    old->first->destroy();
+	  }
+      }
+    }
+
+    formula*
+    mark_tools::mark_concat_ops(const formula* f)
+    {
+      f2f_map::iterator i = markops_.find(f);
+      if (i != markops_.end())
+	return i->second->clone();
+
+      const_cast<formula*>(f)->accept(*markvisitor_);
+
+      formula* r = down_cast<mark_visitor*>(markvisitor_)->result();
+      markops_[f->clone()] = r->clone();
+      return r;
+    }
+
+    formula*
+    mark_tools::simplify_mark(const formula* f)
+    {
+      if (!f->is_marked())
+	return f->clone();
+
+      f2f_map::iterator i = simpmark_.find(f);
+      if (i != simpmark_.end())
+	return i->second->clone();
+
+      const_cast<formula*>(f)->accept(*simpvisitor_);
+
+      formula* r = down_cast<simplify_mark_visitor*>(simpvisitor_)->result();
+      simpmark_[f->clone()] = r->clone();
+      return r;
     }
 
   }
diff --git a/src/ltlvisit/mark.hh b/src/ltlvisit/mark.hh
index 834e14da6..fa5e6c9e1 100644
--- a/src/ltlvisit/mark.hh
+++ b/src/ltlvisit/mark.hh
@@ -1,5 +1,5 @@
-// Copyright (C) 2010 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE).
+// Copyright (C) 2010, 2011 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -23,18 +23,35 @@
 
 #include "ltlast/formula.hh"
 #include "ltlast/visitor.hh"
+#include "misc/hash.hh"
 
 namespace spot
 {
   namespace ltl
   {
-    /// \brief Mark operators UConcat and EConcat.
-    /// \ingroup ltl_rewriting
-    ///
-    /// \param f The formula to rewrite.
-    formula* mark_concat_ops(const formula* f);
+    class mark_tools
+    {
+    public:
+      /// \brief Mark operators UConcat and EConcat.
+      /// \ingroup ltl_rewriting
+      ///
+      /// \param f The formula to rewrite.
+      formula* mark_concat_ops(const formula* f);
+
+      formula* simplify_mark(const formula* f);
+
+      mark_tools();
+      ~mark_tools();
+
+    private:
+      typedef Sgi::hash_map<const formula*, const formula*,
+			    ptr_hash<formula> > f2f_map;
+      f2f_map simpmark_;
+      f2f_map markops_;
+      visitor* simpvisitor_;
+      visitor* markvisitor_;
+    };
 
-    bool simplify_mark(formula*& f);
   }
 }
 
diff --git a/src/tgbaalgos/ltl2tgba_fm.cc b/src/tgbaalgos/ltl2tgba_fm.cc
index a0d2d6d97..59c72052c 100644
--- a/src/tgbaalgos/ltl2tgba_fm.cc
+++ b/src/tgbaalgos/ltl2tgba_fm.cc
@@ -108,6 +108,7 @@ namespace spot
 
       bdd_dict* dict;
       ltl_simplifier* ls;
+      mark_tools mt;
 
       typedef bdd_dict::fv_map fv_map;
       typedef bdd_dict::vf_map vf_map;
@@ -1760,7 +1761,11 @@ namespace spot
 		// Handle a Miyano-Hayashi style unrolling for
 		// rational operators.  Marked nodes correspond to
 		// subformulae in the Miyano-Hayashi set.
-		if (simplify_mark(dest))
+		formula* tmp =  d_.mt.simplify_mark(dest);
+		dest->destroy();
+		dest = tmp;
+
+		if (dest->is_marked())
 		  {
 		    // Make the promise that we will exit marked sets.
 		    int a =
@@ -1772,7 +1777,7 @@ namespace spot
 		    // We have no marked operators, but still
 		    // have other rational operator to check.
 		    // Start a new marked cycle.
-		    formula* dest2 = mark_concat_ops(dest);
+		    formula* dest2 = d_.mt.mark_concat_ops(dest);
 		    dest->destroy();
 		    dest = dest2;
 		  }