diff --git a/ChangeLog b/ChangeLog
index 3f8fc5c7a..071d6070e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2003-05-16  Alexandre Duret-Lutz  <aduret@src.lip6.fr>
 
+	* src/ltlvisit/dump.hh (dump): Return the passed ostream.
+	* src/ltlvisit/dump.cc (dump): Likewise.
+	* src/ltlvisit/dotty.hh (dotty): Likewise.
+	* src/ltlvisit/dotty.cc (dotty): Likewise.
+	* src/ltlvisit/tostring.hh (to_string): Likewise.
+	* src/ltlvisit/tostring.cc (to_string): Likewise.
+
 	* src/ltlvisit/dump.hh (dump): Take a formula* as argument,
 	not a formula&.  This is more homogeneous.
 	* src/ltlvisit/dump.cc (dump): Likewise.
@@ -7,7 +14,7 @@
 	* src/ltlvisit/dotty.cc (dotty): Likewise.
 	* src/ltlvisit/tostring.hh (to_string): Likewise.
 	* src/ltlvisit/tostring.cc (to_string): Likewise.
-	* src/ltltest/readltl.cc, src/ltltest/equals.cc, 
+	* src/ltltest/readltl.cc, src/ltltest/equals.cc,
 	src/ltltest/tostring.cc: Adjust usage.
 
 	Check trivial multop equality at build time.  The makes the
diff --git a/src/ltlvisit/dotty.cc b/src/ltlvisit/dotty.cc
index 741a88a84..581713dca 100644
--- a/src/ltlvisit/dotty.cc
+++ b/src/ltlvisit/dotty.cc
@@ -94,13 +94,14 @@ namespace spot
 
     };
 
-    void
+    std::ostream&
     dotty(const formula* f, std::ostream& os)
     {
       dotty_visitor v(os);
       os << "digraph G {" << std::endl;
       f->accept(v);
       os << "}" << std::endl;
+      return os;
     }
 
   }
diff --git a/src/ltlvisit/dotty.hh b/src/ltlvisit/dotty.hh
index f7de2b09a..6c25b2168 100644
--- a/src/ltlvisit/dotty.hh
+++ b/src/ltlvisit/dotty.hh
@@ -14,7 +14,7 @@ namespace spot
     ///
     /// \c dot is part of the GraphViz package
     /// http://www.research.att.com/sw/tools/graphviz/
-    void dotty(const formula* f, std::ostream& os);
+    std::ostream& dotty(const formula* f, std::ostream& os);
   }
 }
 
diff --git a/src/ltlvisit/dump.cc b/src/ltlvisit/dump.cc
index 4996cf50a..c418a8983 100644
--- a/src/ltlvisit/dump.cc
+++ b/src/ltlvisit/dump.cc
@@ -68,11 +68,12 @@ namespace spot
       std::ostream& os_;
     };
 
-    void
+    std::ostream&
     dump(const formula* f, std::ostream& os)
     {
       dump_visitor v(os);
       f->accept(v);
+      return os;
     }
 
   }
diff --git a/src/ltlvisit/dump.hh b/src/ltlvisit/dump.hh
index 3d0e6ed2e..d6498e2c4 100644
--- a/src/ltlvisit/dump.hh
+++ b/src/ltlvisit/dump.hh
@@ -13,7 +13,7 @@ namespace spot
     /// \param os The stream where it should be output.
     ///
     /// This is useful to display a formula when debugging.
-    void dump(const formula* f, std::ostream& os);
+    std::ostream& dump(const formula* f, std::ostream& os);
   }
 }
 
diff --git a/src/ltlvisit/tostring.cc b/src/ltlvisit/tostring.cc
index 43c141c51..909dc9488 100644
--- a/src/ltlvisit/tostring.cc
+++ b/src/ltlvisit/tostring.cc
@@ -114,11 +114,12 @@ namespace spot
       std::ostream& os_;
     };
 
-    void
+    std::ostream&
     to_string(const formula* f, std::ostream& os)
     {
       to_string_visitor v(os);
       f->accept(v);
+      return os;
     }
 
     std::string
diff --git a/src/ltlvisit/tostring.hh b/src/ltlvisit/tostring.hh
index beb5f6a98..966932b45 100644
--- a/src/ltlvisit/tostring.hh
+++ b/src/ltlvisit/tostring.hh
@@ -8,7 +8,7 @@ namespace spot
 {
   namespace ltl
   {
-    void to_string(const formula* f, std::ostream& os);
+    std::ostream& to_string(const formula* f, std::ostream& os);
     std::string to_string(const formula* f);
   }
 }