Hello *, I am wondering if I should expect some uniqueness guarantees on contained element from fusion set or map containers? In this example: struct print_value { template<class T> void operator()(T& v)const { cout << typeid(T).name() << " has value: " << v << endl; } }; boost::fusion::set s; boost::fusion::for_each(s, print_value()); I get 3 lines - one for each int I put in. I would expect only one line. What are my expectations for at_key? Which int would it return? Which type is going to be returned after I inserted the same type again? May be the better question would be, is the new type inserted as the head element or at the end of the underlying list? Many thanks for the anserws? With Kind Regards, Ovanes