diff --git a/ChangeLog b/ChangeLog
index 1ea5c3265..9b90b64aa 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2004-08-09  Alexandre Duret-Lutz  <adl@src.lip6.fr>
 
+	* src/ltlvisit/apcollect.cc, src/ltlvisit/apcollect.hh: New files,
+	based on code from <Denis.Poitrenaud@lip6.fr>.
+	* src/ltlvisit/Makefile.am (libltlvisit_la_SOURCES, ltlvisit_HEADERS):
+	Add them.
+
 	* iface/gspn/common.cc, iface/gspn/common.hh,
 	src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
 	src/ltlparse/fmterror.cc, src/ltlparse/public.hh,
diff --git a/src/ltlvisit/Makefile.am b/src/ltlvisit/Makefile.am
index 71e0b160e..121fd9c95 100644
--- a/src/ltlvisit/Makefile.am
+++ b/src/ltlvisit/Makefile.am
@@ -25,6 +25,7 @@ AM_CXXFLAGS = $(WARNING_CXXFLAGS)
 ltlvisitdir = $(pkgincludedir)/ltlvisit
 
 ltlvisit_HEADERS = \
+  apcollect.hh \
   basicreduce.hh \
   clone.hh \
   destroy.hh \
@@ -42,6 +43,7 @@ ltlvisit_HEADERS = \
 
 noinst_LTLIBRARIES = libltlvisit.la
 libltlvisit_la_SOURCES = \
+  apcollect.cc \
   basicreduce.cc \
   clone.cc \
   destroy.cc \
diff --git a/src/ltlvisit/apcollect.cc b/src/ltlvisit/apcollect.cc
new file mode 100644
index 000000000..e41532682
--- /dev/null
+++ b/src/ltlvisit/apcollect.cc
@@ -0,0 +1,64 @@
+// Copyright (C) 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
+// département Systèmes Répartis Coopératifs (SRC), Université Pierre
+// et Marie Curie.
+//
+// This file is part of Spot, a model checking library.
+//
+// Spot is free software; you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// Spot is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+// or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+// License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Spot; see the file COPYING.  If not, write to the Free
+// Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+// 02111-1307, USA.
+
+#include "apcollect.hh"
+
+namespace spot
+{
+  namespace ltl
+  {
+    namespace
+    {
+      class atomic_prop_collector : public spot::ltl::postfix_visitor
+      {
+      public:
+	atomic_prop_collector(atomic_prop_set* s)
+	  : postfix_visitor(), sap(s)
+	{
+	}
+
+	virtual ~atomic_prop_collector()
+	{
+	}
+
+	virtual void doit(spot::ltl::atomic_prop* ap)
+	{
+	  sap->insert(ap);
+	}
+
+      private:
+	atomic_prop_set* sap;
+      };
+    }
+
+    atomic_prop_set*
+    atomic_prop_collect(const formula* f, atomic_prop_set* s)
+    {
+      if (!s)
+	s = new atomic_prop_set;
+      atomic_prop_collector v(s);
+      const_cast<formula*>(f)->accept(v);
+      return s;
+    }
+
+  }
+
+}
diff --git a/src/ltlvisit/apcollect.hh b/src/ltlvisit/apcollect.hh
new file mode 100644
index 000000000..e2e651ebd
--- /dev/null
+++ b/src/ltlvisit/apcollect.hh
@@ -0,0 +1,47 @@
+// Copyright (C) 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
+// département Systèmes Répartis Coopératifs (SRC), Université Pierre
+// et Marie Curie.
+//
+// This file is part of Spot, a model checking library.
+//
+// Spot is free software; you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// Spot is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+// or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+// License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Spot; see the file COPYING.  If not, write to the Free
+// Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+// 02111-1307, USA.
+
+#ifndef SPOT_LTLVISIT_APCOLLECT_HH
+# define SPOT_LTLVISIT_AP_COLLECT_HH
+
+#include <set>
+#include "ltlvisit/postfix.hh"
+
+namespace spot
+{
+  namespace ltl
+  {
+    /// Set of atomic propositions.
+    typedef std::set<spot::ltl::atomic_prop*> atomic_prop_set;
+
+    /// \brief Return the set of atomic propositions occurring in a formula.
+    ///
+    /// \param f the formula to inspect
+    /// \param s an existing set to fill with atomic_propositions discovered,
+    ///        or 0 if the set should be allocated by the function.
+    /// \return A pointer to the supplied set, \c s, augmented with
+    ///        atomic propositions occurring in \c f; or a newly allocated
+    ///        set containing all these atomic propositions if \c s is 0.
+    atomic_prop_set*
+    atomic_prop_collect(const formula* f, atomic_prop_set* s = 0);
+  }
+}
+#endif