Ok, I have the recursive equivalent of my original implementation now. About 2/3 in length, perhaps marginally easier to understand. However 2^7 combinations kick it (with VStudio 2008) into never-never-land and the iterative solution takes it in stride, if not instantaneously. So, do you think this thingie belongs in MPL? ...Max... ===================================================================================== template< typename S > struct flatten_view; template< typename S > struct flatten_descend_once { typedef joint_view< typename front<S>::type, flatten_view< iterator_range< typename next< typename begin< S >::type >::type, typename end< S >::type > > > type; }; template< typename S > struct flatten_view : public eval_if< typename empty<S>::type, identity<S>, flatten_descend_once<S> >::type { typedef flatten_view type; }; template< typename S, typename A> struct product_view_ex; template< typename S, typename A > struct product_descend_once { typedef flatten_view< transform_view< typename front<S>::type, product_view_ex< typename pop_front<S>::type, push_front<A,_1> > > > type; }; template< typename S, typename A> struct product_view_ex : public eval_if< typename empty<S>::type, push_back< S, A>, product_descend_once< S, A > >::type { typedef product_view_ex type; }; template< typename S> struct product_view : public if_< typename empty<S>::type, S, product_view_ex< S, typename clear<S>::type > >::type { typedef product_view type; };