diff --git a/ChangeLog b/ChangeLog
index 97d9b9f49..44856bfe9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2011-01-27  Alexandre Duret-Lutz  <adl@lrde.epita.fr>
+
+	Report formulas that are both safety and guarantee.
+
+	* src/tgbatest/ltl2tgba.cc (-O): Report formulas that are both
+	safety and guarantee.
+	* src/tgbatest/obligation.test: Add cases.
+
 2011-01-27  Alexandre Duret-Lutz  <adl@lrde.epita.fr>
 
 	Rename is_safety_automaton() as is_guarantee_automaton() and
diff --git a/src/tgbatest/ltl2tgba.cc b/src/tgbatest/ltl2tgba.cc
index 50a127da9..adf950cb0 100644
--- a/src/tgbatest/ltl2tgba.cc
+++ b/src/tgbatest/ltl2tgba.cc
@@ -1152,16 +1152,23 @@ main(int argc, char** argv)
 		}
 	      else
 		{
-		  if (is_guarantee_automaton(minimized))
+		  bool g = is_guarantee_automaton(minimized);
+		  bool s = is_safety_mwdba(minimized);
+		  if (g && !s)
 		    {
 		      std::cout << "this is a guarantee property (hence, "
 				<< "an obligation property)";
 		    }
-		  else if (is_safety_mwdba(minimized))
+		  else if (s && !g)
 		    {
 		      std::cout << "this is a safety property (hence, "
 				<< "an obligation property)";
 		    }
+		  else if (s && g)
+		    {
+		      std::cout << "this is a guarantee and a safety property"
+				<< " (and of course an obligation property)";
+		    }
 		  else
 		    {
 		      std::cout << "this is an obligation property that is "
diff --git a/src/tgbatest/obligation.test b/src/tgbatest/obligation.test
index 882385a4d..970df7305 100755
--- a/src/tgbatest/obligation.test
+++ b/src/tgbatest/obligation.test
@@ -80,6 +80,9 @@ NO G(q->(p->(!r U (s&!r&X(!r U t))))U(r|G(p->(s&XFt))))
 NO G(p->F(s&!z&X(!z U t)))
 NO G(q->G(p->(s&!z&X(!z U t))))
 NO G(q->(p->(!r U (s&!r&!z&X((!r&!z) U t))))U(r|G(p->(s&!z&X(!z U t)))))
+GS p
+GS q&Xp
+GS G(Ga&F!a)
 EOF
 
 grok()
@@ -87,6 +90,7 @@ grok()
   case $1 in
   "this is a safety property"*)          echo SA;;
   "this is a guarantee property"*)       echo GU;;
+  "this is a guarantee and a safety"*)   echo GS;;
   "this is an obligation property"*)     echo OB;;
   "this is not an obligation property"*) echo NO;;
   *)                                     echo XX;;
@@ -107,6 +111,7 @@ while read exp f; do
   case $resx,$resy in
       SA,GU);;
       GU,SA);;
+      GS,GS);;
       OB,OB);;
       NO,NO);;
       *) echo "Incompatible results: $resx,$resy"; exit 1;;