containment can be serialized

doc/examples/contain_serialization.cxx

@@ -37,6 +37,10 @@ class B: public xml::Serialize {
     }
 };
 
+std::ostream& fn(std::ostream& os) {
+  os<<std::endl<<"HALLO"<<std::endl;
+}
+
 int main(int, char**) {
   { // Serialization as a member
     std::stringstream ss("<b>\n"
@@ -54,6 +58,7 @@ int main(int, char**) {
     std::cout<<"Text B: "<<b.txt<<std::endl;
     std::cout<<"Text A: "<<b.a.txt<<std::endl;
     b.saveXml(std::cout)<<std::endl;
+    std::cout<<fn;
   }
   return 0;
 }