The documentation for push_back specifies a return type of result_of::push_back<Sequence, T>::type where Sequence is not const-qualified: template< typename Sequence, typename T > typename result_of::push_back<Sequence, T>::type push_back( Sequence const& seq, T const& t); yet the implementation is defined to return result_of::push_back<Sequence const, T>::type, note the const-qualified Sequence: template <typename Sequence, typename T> inline typename lazy_enable_if< traits::is_sequence<Sequence> , result_of::push_back<Sequence const, T> >::type push_back(Sequence const& seq, T const& x) Is this a documentation bug? Agustín K-ballo Bergé.- http://fusionfenix.com