[Boost-users] MPL. Difference between variadic and numbered forms of sequences

When I write expressions like: template<class Seq, class Elem> struct pairs: fold<Seq, vector<>, push_back<_1,vector2<Elem, _2 > > >{}; Everything works just fine, but when I use variadic form: template<class Seq, class Elem> struct pairs: fold<Seq, vector<>, push_back<_1,vector<Elem, _2 > > >{}; I have a lot of errors. Can someone explain me what's the difference?