[boost] mpl::vector & push_back question ?

Hi: When I am reading mpl source code. I notice that push_bck algorithm, Look as following: template<> struct push_back_traits< aux::vector_tag > { template< typename Vector, typename T > struct algorithm { typedef vector_node<T,Vector> type; }; }; My question is: when generate new sequence, vactor_node<T, vector>, why skip "size" argument ? How does it increase sequence size in push_back? About push_front, It increase sequence size when generating new sequence. Regard Karl