diff --git a/Surface_mesh_topology/examples/Surface_mesh_topology/CMakeLists.txt b/Surface_mesh_topology/examples/Surface_mesh_topology/CMakeLists.txt
index 31a82ebc13d..d971511e0b8 100644
--- a/Surface_mesh_topology/examples/Surface_mesh_topology/CMakeLists.txt
+++ b/Surface_mesh_topology/examples/Surface_mesh_topology/CMakeLists.txt
@@ -10,6 +10,8 @@ if(CGAL_Qt5_FOUND)
     add_definitions(-DCGAL_USE_BASIC_VIEWER -DQT_NO_KEYWORDS)
 endif()
 
+# add_definitions(-DCGAL_TRACE_PATH_TESTS)
+
 if ( CGAL_FOUND )
 
   # create a target per cppfile
diff --git a/Surface_mesh_topology/examples/Surface_mesh_topology/path_homotopy.cpp b/Surface_mesh_topology/examples/Surface_mesh_topology/path_homotopy.cpp
index f178365f450..ebbd6a1665c 100644
--- a/Surface_mesh_topology/examples/Surface_mesh_topology/path_homotopy.cpp
+++ b/Surface_mesh_topology/examples/Surface_mesh_topology/path_homotopy.cpp
@@ -126,6 +126,10 @@ int main(int argc, char** argv)
 
   for (unsigned int i=0; i<N; ++i)
   {
+    // TEMPO POUR DEBUG
+    //random=CGAL::Random(461974893);
+    // END TEMPO POUR DEBUG
+
     if (i!=0)
     {
       random=CGAL::Random(random.get_int(0, std::numeric_limits<int>::max()));
@@ -150,8 +154,7 @@ int main(int argc, char** argv)
     CGAL::Path_on_surface<LCC_3_cmap> path1(lcc);
     path1.generate_random_closed_path(F, random);
 
-    //if (path1.length()<100000) // TEMPO FOR DEBUG
-    {
+    //if (path1.length()<100000) { // TEMPO FOR DEBUG
     std::cout<<"Path1 size: "<<path1.length()<<" (from "<<F<<" darts); ";
     paths.push_back(path1);
 
@@ -164,7 +167,8 @@ int main(int argc, char** argv)
     if (smct.is_contractible(path1, time))
     { ++nbcontractible; }
 
-    if (!smct.are_freely_homotopic(path1, path2, time))
+    bool res=smct.are_freely_homotopic(path1, path2, time);
+    if (!res)
     {
       /* std::cout<<"ERROR: paths are not homotopic while they should be."
                <<std::endl; */
@@ -177,8 +181,9 @@ int main(int argc, char** argv)
     if (draw)
     { CGAL::draw(lcc, paths); }
 #endif
-    }
-    // else { --i; } // TEMPO POUR DEBUG
+     // END TEMPO POUR DEBUG}
+
+   //  { if (!res) { i=0; errors_seeds.clear();} } // TEMPO POUR DEBUG
   }
 
   if (errors_seeds.empty())
diff --git a/Surface_mesh_topology/include/CGAL/Path_on_surface_with_rle.h b/Surface_mesh_topology/include/CGAL/Path_on_surface_with_rle.h
index b792691a79d..51f924565b5 100644
--- a/Surface_mesh_topology/include/CGAL/Path_on_surface_with_rle.h
+++ b/Surface_mesh_topology/include/CGAL/Path_on_surface_with_rle.h
@@ -328,6 +328,12 @@ public:
     return dhend==flat->end;
   }
 
+  void display_flat(std::ostream& os, const List_iterator& flat)
+  {
+    os<<"["<<m_map.darts().index(flat->begin)<<", "
+      <<m_map.darts().index(flat->end)<<" (l="<<flat->length<<")]";
+  }
+
   /// @return true iff the path is valid; i.e. a sequence of flats two by
   ///              two adjacent.
   /// if test_minimal is true, test that there is no two consecutive flats
@@ -977,6 +983,12 @@ public:
     if (!can_merge_with_next_flat(it, positive2, negative2))
     { return false; }
 
+#ifdef CGAL_TRACE_PATH_TESTS
+        std::cout<<"Merge with next flat: ";
+        display_flat(std::cout, it); std::cout<<" and ";
+        display_flat(std::cout, next_iterator(it)); std::cout<<std::endl;
+#endif
+
     List_iterator it2=next_iterator(it);
     set_flat_length(it, flat_length(it)+flat_length(it2)+
                     (positive2?1:-1));
@@ -1005,6 +1017,12 @@ public:
   /// ('it' will be equal to m_path.end() if the path becomes empty).
   void remove_spur(List_iterator& it)
   {
+#ifdef CGAL_TRACE_PATH_TESTS
+        std::cout<<"Remove spur between flats: ";
+        display_flat(std::cout, it); std::cout<<" and ";
+        display_flat(std::cout, next_iterator(it)); std::cout<<std::endl;
+#endif
+
     CGAL_assertion(is_spur(it));
     Set_of_it modified_flats;
 
@@ -1145,6 +1163,13 @@ public:
   /// of the bracket; it3 is the flat end of the bracket.
   void remove_negative_bracket(List_iterator& it1, List_iterator& it3)
   {
+#ifdef CGAL_TRACE_PATH_TESTS
+    std::cout<<"Remove negative bracket: ";
+    display_flat(std::cout, it1); std::cout<<", ";
+    display_flat(std::cout, next_iterator(it1)); std::cout<<" and ";
+    display_flat(std::cout, it3); std::cout<<std::endl;
+#endif
+
     Set_of_it modified_flats;
     List_iterator it2;
     CGAL_assertion(is_negative_bracket(it1, it2)); // Here it2 is unused
@@ -1192,6 +1217,13 @@ public:
   void remove_positive_bracket(List_iterator& it1,
                                List_iterator& it3)
   {
+#ifdef CGAL_TRACE_PATH_TESTS
+    std::cout<<"Remove positive bracket: ";
+    display_flat(std::cout, it1); std::cout<<", ";
+    display_flat(std::cout, next_iterator(it1)); std::cout<<" and ";
+    display_flat(std::cout, it3); std::cout<<std::endl;
+#endif
+
     Set_of_it modified_flats;
     List_iterator it2;
     CGAL_assertion(is_positive_bracket(it1, it2)); // Here it2 is unused
@@ -1401,7 +1433,14 @@ public:
   
   /// Right push the given l-shape.
   void right_push_l_shape(List_iterator& it1)
-  { // it is the beginning of a flat
+  {
+#ifdef CGAL_TRACE_PATH_TESTS
+        std::cout<<"Right push l-shape: ";
+        display_flat(std::cout, it1); std::cout<<" and ";
+        display_flat(std::cout, next_iterator(it1)); std::cout<<std::endl;
+#endif
+
+    // it is the beginning of a flat
     CGAL_assertion(is_l_shape(it1));
     // CGAL_assertion(is_valid());
     Set_of_it modified_flats;