diff --git a/ChangeLog b/ChangeLog
index 502386768..52a837f64 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2004-11-02  Alexandre Duret-Lutz  <adl@src.lip6.fr>
+
+	* src/ltlast/formula.hh (ltl::formula::~formula): Make it protected.
+
 2004-10-29  Alexandre Duret-Lutz  <adl@src.lip6.fr>
 
 	A tgba can now annotate a transition (i.e., the position of a
diff --git a/src/ltlast/formula.hh b/src/ltlast/formula.hh
index 6d15b8449..40a6ef221 100644
--- a/src/ltlast/formula.hh
+++ b/src/ltlast/formula.hh
@@ -38,8 +38,6 @@ namespace spot
     class formula
     {
     public:
-      virtual ~formula();
-
       /// Entry point for vspot::ltl::visitor instances.
       virtual void accept(visitor& v) = 0;
       /// Entry point for vspot::ltl::const_visitor instances.
@@ -61,6 +59,8 @@ namespace spot
       static void unref(formula* f);
 
     protected:
+      virtual ~formula();
+
       /// \brief increment reference counter if any
       virtual void ref_();
       /// \brief decrement reference counter if any, return true when