template<class T> struct L2 { typedef bool is_kdtree_distance; typedef T ElementType; typedef typename Accumulator<T>::Type ResultType; template <typename Iterator1, typename Iterator2> ResultType ...

Notice the ellipsis in the definition above. This is a new C++11 feature known as "variadic templates". My function gets at least two parameters -- friend1 and friend2 -- both of a same generic type, ...