diff --git a/src/ltlvisit/lunabbrev.cc b/src/ltlvisit/lunabbrev.cc
index 30ccf50da..7bc002b90 100644
--- a/src/ltlvisit/lunabbrev.cc
+++ b/src/ltlvisit/lunabbrev.cc
@@ -106,6 +106,8 @@ namespace spot
     formula*
     unabbreviate_logic(const formula* f)
     {
+      if (f->is_sugar_free_boolean())
+	return f->clone();
       unabbreviate_logic_visitor v;
       const_cast<formula*>(f)->accept(v);
       return v.result();
diff --git a/src/ltlvisit/nenoform.cc b/src/ltlvisit/nenoform.cc
index 294a0da1a..1392b41b1 100644
--- a/src/ltlvisit/nenoform.cc
+++ b/src/ltlvisit/nenoform.cc
@@ -289,6 +289,8 @@ namespace spot
     formula*
     negative_normal_form(const formula* f, bool negated)
     {
+      if (!negated && f->is_in_nenoform())
+	return f->clone();
       negative_normal_form_visitor v(negated);
       const_cast<formula*>(f)->accept(v);
       return v.result();
diff --git a/src/ltlvisit/simpfg.cc b/src/ltlvisit/simpfg.cc
index b6567e282..498bae629 100644
--- a/src/ltlvisit/simpfg.cc
+++ b/src/ltlvisit/simpfg.cc
@@ -98,6 +98,8 @@ namespace spot
     formula*
     simplify_f_g(const formula* f)
     {
+      if (f->is_boolean())
+	return f->clone();
       simplify_f_g_visitor v;
       const_cast<formula*>(f)->accept(v);
       return v.result();
diff --git a/src/ltlvisit/tunabbrev.cc b/src/ltlvisit/tunabbrev.cc
index 21c65d59a..7bbcd6cc2 100644
--- a/src/ltlvisit/tunabbrev.cc
+++ b/src/ltlvisit/tunabbrev.cc
@@ -70,6 +70,8 @@ namespace spot
     formula*
     unabbreviate_ltl(const formula* f)
     {
+      if (f->is_sugar_free_boolean() && f->is_sugar_free_ltl())
+	return f->clone();
       unabbreviate_ltl_visitor v;
       const_cast<formula*>(f)->accept(v);
       return v.result();