ltl: rename is_X_free() into is_syntactic_stutter_invariant()

and adjust it to detect siPSL formulas, as in the paper of Dax et
al. (ATVA'09).  For issue #51.

* src/ltlast/atomic_prop.cc, src/ltlast/binop.cc,
src/ltlast/bunop.cc, src/ltlast/constant.cc, src/ltlast/formula.cc,
src/ltlast/formula.hh, src/ltlast/multop.cc, src/ltlast/unop.cc: Rename
the property, and adjust its computation on siSERE.
* src/ltlvisit/remove_x.cc, src/ltlvisit/simplify.cc,
src/tgbaalgos/stutter.cc: Adjust to new names.
* src/bin/ltlfilt.cc: Add option --syntactic-sutter-invariant.
* src/ltltest/kind.test: Update tests and add some new.

src/bin/ltlfilt.cc

@@ -58,7 +58,7 @@ Exit status:\n\
 #define OPT_DROP_ERRORS 2
 #define OPT_NNF 3
 #define OPT_LTL 4
-#define OPT_NOX 5
+#define OPT_SYNTACTIC_SI 5
 #define OPT_BOOLEAN 6
 #define OPT_EVENTUAL 7
 #define OPT_UNIVERSAL 8
@@ -121,7 +121,6 @@ static const argp_option options[] =
     { 0, 0, 0, 0,
       "Filtering options (matching is done after transformation):", 5 },
     { "ltl", OPT_LTL, 0, 0, "match only LTL formulas (no PSL operator)", 0 },
-    { "nox", OPT_NOX, 0, 0, "match X-free formulas", 0 },
     { "boolean", OPT_BOOLEAN, 0, 0, "match Boolean formulas", 0 },
     { "eventual", OPT_EVENTUAL, 0, 0, "match pure eventualities", 0 },
     { "universal", OPT_UNIVERSAL, 0, 0, "match purely universal formulas", 0 },
@@ -135,6 +134,9 @@ static const argp_option options[] =
       "match syntactic-recurrence formulas", 0 },
     { "syntactic-persistence", OPT_SYNTACTIC_PERSISTENCE, 0, 0,
       "match syntactic-persistence formulas", 0 },
+    { "syntactic-stutter-invariant", OPT_SYNTACTIC_SI, 0, 0,
+      "match stutter-invariant formulas syntactically (LTL-X or siPSL)", 0 },
+    { "nox", 0, 0, OPTION_ALIAS, 0, 0 },
     { "safety", OPT_SAFETY, 0, 0,
       "match safety formulas (even pathological)", 0 },
     { "guarantee", OPT_GUARANTEE, 0, 0,
@@ -206,7 +208,6 @@ static bool boolean_to_isop = false;
 static bool unique = false;
 static bool psl = false;
 static bool ltl = false;
-static bool nox = false;
 static bool invert = false;
 static bool boolean = false;
 static bool universal = false;
@@ -216,6 +217,7 @@ static bool syntactic_guarantee = false;
 static bool syntactic_obligation = false;
 static bool syntactic_recurrence = false;
 static bool syntactic_persistence = false;
+static bool syntactic_si = false;
 static bool safety = false;
 static bool guarantee = false;
 static bool obligation = false;
@@ -349,9 +351,6 @@ parse_opt(int key, char* arg, struct argp_state*)
     case OPT_NNF:
       nnf = true;
       break;
-    case OPT_NOX:
-      nox = true;
-      break;
     case OPT_OBLIGATION:
       obligation = true;
       break;
@@ -407,6 +406,9 @@ parse_opt(int key, char* arg, struct argp_state*)
     case OPT_SYNTACTIC_PERSISTENCE:
       syntactic_persistence = true;
       break;
+    case OPT_SYNTACTIC_SI:
+      syntactic_si = true;
+      break;
     case OPT_UNIVERSAL:
       universal = true;
       break;
@@ -558,7 +560,6 @@ namespace
 
       matched &= !ltl || f->is_ltl_formula();
       matched &= !psl || f->is_psl_formula();
-      matched &= !nox || f->is_X_free();
       matched &= !boolean || f->is_boolean();
       matched &= !universal || f->is_universal();
       matched &= !eventual || f->is_eventual();
@@ -567,6 +568,7 @@ namespace
       matched &= !syntactic_obligation || f->is_syntactic_obligation();
       matched &= !syntactic_recurrence || f->is_syntactic_recurrence();
       matched &= !syntactic_persistence || f->is_syntactic_persistence();
+      matched &= !syntactic_si || f->is_syntactic_stutter_invariant();
       matched &= !ap || atomic_prop_collect(f)->size() == ap_n;
 
       if (matched && (size_min > 0 || size_max >= 0))

src/ltlast/atomic_prop.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014 Laboratoire de
+// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014, 2015 Laboratoire de
 // Recherche et Développement de l'Epita (LRDE).
 // Copyright (C) 2003, 2004, 2005 Laboratoire d'Informatique de
 // Paris 6 (LIP6), département Systèmes Répartis Coopératifs (SRC),
@@ -38,7 +38,10 @@ namespace spot
       is.boolean = true;
       is.sugar_free_boolean = true;
       is.in_nenoform = true;
-      is.X_free = true;
+      is.syntactic_si = true;	// Assuming LTL (for PSL a Boolean
+				// term is not stared will be regarded
+				// as not stuttering were this
+				// matters.)
       is.sugar_free_ltl = true;
       is.ltl_formula = true;
       is.eltl_formula = true;

src/ltlast/binop.cc

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014 Laboratoire de
+// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014, 2015 Laboratoire
-// Recherche et Développement de l'Epita (LRDE).
+// de Recherche et Développement de l'Epita (LRDE).
 // Copyright (C) 2003, 2005 Laboratoire d'Informatique de Paris
 // 6 (LIP6), département Systèmes Répartis Coopératifs (SRC),
 // Université Pierre et Marie Curie.
@@ -132,6 +132,8 @@ namespace spot
 	    }
 	  assert(first->is_sere_formula());
 	  assert(second->is_psl_formula());
+	  if (first->is_boolean())
+	    is.syntactic_si = false;
 	  break;
 	case UConcat:
 	  is.not_marked = true;
@@ -158,6 +160,8 @@ namespace spot
 	    }
 	  assert(first->is_sere_formula());
 	  assert(second->is_psl_formula());
+	  if (first->is_boolean())
+	    is.syntactic_si = false;
 	  break;
 	case U:
 	  is.not_marked = true;

src/ltlast/bunop.cc

@@ -57,14 +57,28 @@ namespace spot
 	{
 	case Star:
 	  if (max_ == unbounded)
-	    is.finite = false;
+	    {
+	      is.finite = false;
+	      is.syntactic_si = min_ == 1 && child->is_boolean();
+	    }
+	  else
+	    {
+	      is.syntactic_si = false;
+	    }
 	  if (min_ == 0)
 	    is.accepting_eword = true;
 	  break;
 	case FStar:
 	  is.accepting_eword = false;
 	  if (max_ == unbounded)
-	    is.finite = false;
+	    {
+	      is.finite = false;
+	      is.syntactic_si &= !child->is_boolean();
+	    }
+	  else
+	    {
+	      is.syntactic_si = false;
+	    }
 	  break;
 	}
     }

src/ltlast/constant.cc

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014 Laboratoire de
+// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014, 2015 Laboratoire
-// Recherche et Développement de l'Epita (LRDE).
+// de Recherche et Développement de l'Epita (LRDE).
 // Copyright (C) 2003, 2005 Laboratoire d'Informatique de Paris
 // 6 (LIP6), département Systèmes Répartis Coopératifs (SRC),
 // Université Pierre et Marie Curie.
@@ -43,7 +43,7 @@ namespace spot
 	  is.boolean = true;
 	  is.sugar_free_boolean = true;
 	  is.in_nenoform = true;
-	  is.X_free = true;
+	  is.syntactic_si = true; // for LTL (not PSL)
 	  is.sugar_free_ltl = true;
 	  is.ltl_formula = true;
 	  is.eltl_formula = true;
@@ -64,7 +64,7 @@ namespace spot
 	  is.boolean = false;
 	  is.sugar_free_boolean = false;
 	  is.in_nenoform = true;
-	  is.X_free = true;
+	  is.syntactic_si = true;
 	  is.sugar_free_ltl = true;
 	  is.ltl_formula = false;
 	  is.eltl_formula = false;

src/ltlast/formula.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014 Laboratoire de
+// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014, 2015 Laboratoire de
 // Recherche et Développement de l'Epita (LRDE).
 // Copyright (C) 2003, 2005 Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
@@ -39,7 +39,8 @@ namespace spot
       proprint(is_boolean, "B", "Boolean formula");			\
       proprint(is_sugar_free_boolean, "&", "without Boolean sugar");	\
       proprint(is_in_nenoform, "!", "in negative normal form");		\
-      proprint(is_X_free, "x", "without X operator");			\
+      proprint(is_syntactic_stutter_invariant, "x",			\
+	       "syntactic stutter invariant");				\
       proprint(is_sugar_free_ltl, "f", "without LTL sugar");		\
       proprint(is_ltl_formula, "L", "LTL formula");			\
       proprint(is_eltl_formula, "E", "ELTL formula");			\

src/ltlast/formula.hh

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2008, 2009, 2010, 2011, 2012, 2013, 2014 Laboratoire
+// Copyright (C) 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015
-// de Recherche et Développement de l'Epita (LRDE).
+// Laboratoire de Recherche et Développement de l'Epita (LRDE).
 // Copyright (C) 2003, 2004, 2005 Laboratoire d'Informatique de
 //
 // This file is part of Spot, a model checking library.
@@ -153,10 +153,10 @@ namespace spot
 	return is.in_nenoform;
       }
 
-      /// Whether the formula avoids the X operator.
+      /// Whether the formula is syntactically stutter_invariant
-      bool is_X_free() const
+      bool is_syntactic_stutter_invariant() const
       {
-	return is.X_free;
+	return is.syntactic_si;
       }
 
       /// Whether the formula avoids the F and G operators.
@@ -337,7 +337,7 @@ namespace spot
 	bool boolean:1;		   // No temporal operators.
 	bool sugar_free_boolean:1; // Only AND, OR, and NOT operators.
 	bool in_nenoform:1;	   // Negative Normal Form.
-	bool X_free:1;		   // No X operators.
+	bool syntactic_si:1;	   // LTL-X or siPSL
 	bool sugar_free_ltl:1;	   // No F and G operators.
 	bool ltl_formula:1;	   // Only LTL operators.
 	bool eltl_formula:1;	   // Only ELTL operators.

src/ltlast/multop.cc

@@ -50,34 +50,61 @@ namespace spot
 	case Concat:
 	case AndNLM:
 	case AndRat:
-	  // Note: AndNLM(p1,p2) and AndRat(p1,p2) are Boolean
+	  {
-	  // formulae, but there are actually rewritten as And(p1,p2)
+	    bool syntactic_si = is.syntactic_si && !is.boolean;
-	  // by trivial identities before this constructor is called.
+	    // Note: AndNLM(p1,p2) and AndRat(p1,p2) are Boolean
-	  // So at this point, AndNLM/AndRat are always used with at
+	    // formulae, but they are actually rewritten as And(p1,p2)
-	  // most one Boolean argument, and the result is therefore
+	    // by trivial identities before this constructor is called.
-	  // NOT Boolean.
+	    // So at this point, AndNLM/AndRat are always used with at
-	  is.boolean = false;
+	    // most one Boolean argument, and the result is therefore
-	  is.ltl_formula = false;
+	    // NOT Boolean.
-	  is.eltl_formula = false;
+	    is.boolean = false;
-	  is.psl_formula = false;
+	    is.ltl_formula = false;
-	  is.eventual = false;
+	    is.eltl_formula = false;
-	  is.universal = false;
+	    is.psl_formula = false;
+	    is.eventual = false;
+	    is.universal = false;
+
+	    for (unsigned i = 1; i < s; ++i)
+	      {
+		syntactic_si &= (*v)[i]->is_syntactic_stutter_invariant()
+		  && !(*v)[i]->is_boolean();
+		props &= (*v)[i]->get_props();
+	      }
+	    is.syntactic_si = syntactic_si;
+	    break;
+	  }
 	case And:
 	  for (unsigned i = 1; i < s; ++i)
 	    props &= (*v)[i]->get_props();
 	  break;
 	case OrRat:
-	  // Note: OrRat(p1,p2) is a Boolean formula, but its is
+	  {
-	  // actually rewritten as Or(p1,p2) by trivial identities
+	    bool syntactic_si = is.syntactic_si && !is.boolean;
-	  // before this constructor is called.  So at this point,
+	    // Note: OrRat(p1,p2) is a Boolean formula, but its is
-	  // AndNLM is always used with at most one Boolean argument,
+	    // actually rewritten as Or(p1,p2) by trivial identities
-	  // and the result is therefore NOT Boolean.
+	    // before this constructor is called.  So at this point,
-	  is.boolean = false;
+	    // AndNLM is always used with at most one Boolean argument,
-	  is.ltl_formula = false;
+	    // and the result is therefore NOT Boolean.
-	  is.eltl_formula = false;
+	    is.boolean = false;
-	  is.psl_formula = false;
+	    is.ltl_formula = false;
-	  is.eventual = false;
+	    is.eltl_formula = false;
-	  is.universal = false;
+	    is.psl_formula = false;
+	    is.eventual = false;
+	    is.universal = false;
+
+	    bool ew = (*v)[0]->accepts_eword();
+	    for (unsigned i = 1; i < s; ++i)
+	      {
+		ew |= (*v)[i]->accepts_eword();
+		syntactic_si &= (*v)[i]->is_syntactic_stutter_invariant()
+		  && !(*v)[i]->is_boolean();
+		props &= (*v)[i]->get_props();
+	      }
+	    is.accepting_eword = ew;
+	    is.syntactic_si = syntactic_si;
+	    break;
+	  }
 	case Or:
 	  {
 	    bool ew = (*v)[0]->accepts_eword();
@@ -90,6 +117,32 @@ namespace spot
 	    break;
 	  }
 	}
+      // A concatenation is an siSERE if it contains one stared
+      // Boolean, and the other operands are siSERE (i.e.,
+      // sub-formulas that verify is_syntactic_stutter_invariant() and
+      // !is_boolean());
+      if (op == Concat)
+	{
+	  unsigned sb = 0; // stared Boolean formulas seen
+	  for (unsigned i = 0; i < s; ++i)
+	    {
+	      if ((*v)[i]->is_syntactic_stutter_invariant()
+		  && !(*v)[i]->is_boolean())
+		continue;
+	      if (const bunop* b = is_Star((*v)[i]))
+		{
+		  sb += b->child()->is_boolean();
+		  if (sb > 1)
+		    break;
+		}
+	      else
+		{
+		  sb = 0;
+		  break;
+		}
+	    }
+	  is.syntactic_si = sb == 1;
+	}
     }
 
     multop::~multop()

src/ltlast/unop.cc

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014 Laboratoire de
+// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014, 2015 Laboratoire
-// Recherche et Développement de l'Epita (LRDE).
+// de Recherche et Développement de l'Epita (LRDE).
 // Copyright (C) 2003, 2005 Laboratoire d'Informatique de Paris
 // 6 (LIP6), département Systèmes Répartis Coopératifs (SRC),
 // Université Pierre et Marie Curie.
@@ -58,7 +58,7 @@ namespace spot
 	case X:
 	  is.not_marked = true;
 	  is.boolean = false;
-	  is.X_free = false;
+	  is.syntactic_si = false;
 	  is.eltl_formula = false;
 	  is.sere_formula = false;
 	  // is.syntactic_safety inherited
@@ -112,6 +112,8 @@ namespace spot
 	  is.syntactic_recurrence = true;
 	  is.syntactic_persistence = true;
 	  is.accepting_eword = false;
+	  assert(child->is_sere_formula());
+	  assert(!child->is_boolean());
 	  break;
 	case Closure:
 	  is.not_marked = true;
@@ -127,6 +129,7 @@ namespace spot
 	  is.syntactic_persistence = true;
 	  is.accepting_eword = false;
 	  assert(child->is_sere_formula());
+	  assert(!child->is_boolean());
 	  break;
 	}
     }

src/ltltest/kind.test

@@ -59,32 +59,47 @@ check '(Ga|Fc) -> Fb'  'xLPopr'
 check '(Ga|Fa) -> Gb'  'xLPopr'
 check '{a;c*;b}|->!Xb' '&fPsopr'
 check '{a;c*;b}|->X!b' '&!fPsopr'
-check '{a;c*;b}|->!Fb' '&xPsopr'
+check '{a;c*;b}|->!Fb' '&Psopr'
-check '{a;c*;b}|->G!b' '&!xPsopr'
+check '{a;c*;b}|->G!b' '&!Psopr'
-check '{a;c*;b}|->!Gb' '&xPr'
+check '{a;c*;b}|->!Gb' '&Pr'
-check '{a;c*;b}|->F!b' '&!xPr'
+check '{a;c*;b}|->F!b' '&!Pr'
-check '{a;c*;b}|->GFa' '&!xPr'
+check '{a;c*;b}|->GFa' '&!Pr'
-check '{a;c*;b}|->FGa' '&!xP'
+check '{a;c*;b}|->FGa' '&!P'
+check '{a[+];c[+];b*}|->!Fb' '&xPsopr'
+check '{a[+];c*;b[+]}|->G!b' '&!xPsopr'
+check '{a*;c[+];b[+]}|->!Gb' '&xPr'
+check '{a[+];c*;b[+]}|->F!b' '&!xPr'
+check '{a[+];c[+];b*}|->GFa' '&!xPr'
+check '{a*;c[+];b[+]}|->FGa' '&!xP'
 check '{a;c;b|(d;e)}|->!Xb' '&fPFsgopr'
 check '{a;c;b|(d;e)}|->X!b' '&!fPFsgopr'
-check '{a;c;b|(d;e)}|->!Fb' '&xPsopr'
+check '{a;c;b|(d;e)}|->!Fb' '&Psopr'
-check '{a;c;b|(d;e)}|->G!b' '&!xPsopr'
+check '{a;c;b|(d;e)}|->G!b' '&!Psopr'
-check '{a;c;b|(d;e)}|->!Gb' '&xPgopr'
+check '{a;c;b|(d;e)}|->!Gb' '&Pgopr'
-check '{a;c;b|(d;e)}|->F!b' '&!xPgopr'
+check '{a;c;b|(d;e)}|->F!b' '&!Pgopr'
-check '{a;c;b|(d;e)}|->GFa' '&!xPr'
+check '{a;c;b|(d;e)}|->GFa' '&!Pr'
-check '{a;c;b|(d;e)}|->FGa' '&!xPp'
+check '{a;c;b|(d;e)}|->FGa' '&!Pp'
-check '{a;c*;b}<>->!Gb' '&xPgopr'
+check '{a[+] && c[+]}|->!Xb' '&fPsopr'
-check '{a;c*;b}<>->F!b' '&!xPgopr'
+check '{a[+] && c[+]}|->X!b' '&!fPsopr'
-check '{a;c*;b}<>->FGb' '&!xPp'
+check '{a[+] && c[+]}|->!Fb' '&xPsopr'
-check '{a;c*;b}<>->!GFb' '&xPp'
+check '{a[+] && c[+]}|->G!b' '&!xPsopr'
-check '{a;c*;b}<>->GFb' '&!xP'
+check '{a[+] && c[+]}|->!Gb' '&xPr'
-check '{a;c*;b}<>->!FGb' '&xP'
+check '{a[+] && c[+]}|->F!b' '&!xPr'
-check '{a;c|d;b}<>->!Gb' '&xPgopr'
+check '{a[+] && c[+]}|->GFa' '&!xPr'
-check '{a;c|d;b}<>->G!b' '&!xPsopr'
+check '{a[+] && c[+]}|->FGa' '&!xP'
-check '{a;c|d;b}<>->FGb' '&!xPp'
+check '{a;c*;b}<>->!Gb' '&Pgopr'
-check '{a;c|d;b}<>->!GFb' '&xPp'
+check '{a;c*;b}<>->F!b' '&!Pgopr'
-check '{a;c|d;b}<>->GFb' '&!xPr'
+check '{a;c*;b}<>->FGb' '&!Pp'
-check '{a;c|d;b}<>->!FGb' '&xPr'
+check '{a;c*;b}<>->!GFb' '&Pp'
+check '{a;c*;b}<>->GFb' '&!P'
+check '{a;c*;b}<>->!FGb' '&P'
+check '{a*;c[+];b[+]}<>->!FGb' '&xP'
+check '{a;c|d;b}<>->!Gb' '&Pgopr'
+check '{a;c|d;b}<>->G!b' '&!Psopr'
+check '{a;c|d;b}<>->FGb' '&!Pp'
+check '{a;c|d;b}<>->!GFb' '&Pp'
+check '{a;c|d;b}<>->GFb' '&!Pr'
+check '{a;c|d;b}<>->!FGb' '&Pr'
 check '{a:b:c:d}!' 'B&!xfLEPSFsgopr'  # Equivalent to a&b&c&d
 check 'a&b&c&d'    'B&!xfLEPSFsgopr'
 check '(Xa <-> XXXc) U (b & Fe)' 'LPgopr'
@@ -110,11 +125,15 @@ check 'FGa W FGb' '&!xLPeu'
 check 'Ga  W FGb' '&!xLPup'
 check 'Ga  W b'   '&!xLPsopr'
 check 'Fa  M b'   '&!xLPgopr'
-check '{a;b*;c}' '&!xfPsopr'
+check '{a;b*;c}' '&!fPsopr'
-check '{a;b*;c}!' '&!xfPgopr'
+check '{a;b*;c}!' '&!fPgopr'
-check '!{a;b*;c}!' '&xfPsopr'  # The negative normal form is {a;b*;c}[]->0
+check '!{a;b*;c}!' '&fPsopr'  # The negative normal form is {a;b*;c}[]->0
-check '{a;b*;c}[]->0' '&!xfPsopr'
+check '{a;b*;c}[]->0' '&!fPsopr'
-check '!{a;b*;c}' '&!xfPgopr'
+check '!{a;b*;c}' '&!fPgopr'
+check '!{a[+];b*;c[+]}' '&!xfPgopr'
+check '{a[+];b*;c[+]}' '&!xfPsopr'
+check '{a[+] && b || c[+]}' '&!fPsopr'
+check '{a[+] && b[+] || c[+]}' '&!xfPsopr'
 
 run 0 ../consterm '1'
 run 0 ../consterm '0'

src/ltlvisit/remove_x.cc

@@ -104,7 +104,7 @@ namespace spot
 
 	virtual const formula* recurse(const formula* f)
 	{
-	  if (f->is_X_free())
+	  if (f->is_syntactic_stutter_invariant())
 	    return f->clone();
 	  f->accept(*this);
 	  return this->result();

src/ltlvisit/simplify.cc

@@ -2805,7 +2805,7 @@ namespace spot
 		    // a & (Xa U b) = b M a
 		    // a & (b | X(b R a)) = b R a
 		    // a & (b | X(b M a)) = b M a
-		    if (!mo->is_X_free())
+		    if (!mo->is_syntactic_stutter_invariant()) // Skip if no X.
 		      {
 			typedef std::unordered_set<const formula*,
 						   ptr_hash<formula>> fset_t;
@@ -2825,7 +2825,7 @@ namespace spot
 			  {
 			    if (!(*res)[n])
 			      continue;
-			    if ((*res)[n]->is_X_free())
+			    if ((*res)[n]->is_syntactic_stutter_invariant())
 			      continue;
 
 			    const formula* xarg = is_XWU((*res)[n]);
@@ -3530,7 +3530,7 @@ namespace spot
 		    // a | (Xa M b) = b U a
 		    // a | (b & X(b W a)) = b W a
 		    // a | (b & X(b U a)) = b U a
-		    if (!mo->is_X_free())
+		    if (!mo->is_syntactic_stutter_invariant()) // Skip if no X
 		      {
 			typedef std::unordered_set<const formula*,
 						   ptr_hash<formula>> fset_t;
@@ -3551,7 +3551,7 @@ namespace spot
 			  {
 			    if (!(*res)[n])
 			      continue;
-			    if ((*res)[n]->is_X_free())
+			    if ((*res)[n]->is_syntactic_stutter_invariant())
 			      continue;
 
 			    const formula* xarg = is_XRM((*res)[n]);

src/tgbaalgos/stutter.cc

@@ -517,7 +517,7 @@ namespace spot
   bool
   is_stutter_invariant(const ltl::formula* f)
   {
-    if (f->is_ltl_formula() && f->is_X_free())
+    if (f->is_ltl_formula() && f->is_syntactic_stutter_invariant())
       return true;
 
     int algo = default_stutter_check_algorithm();