How does tuple

When I read post Variadic templates in C++, I could not get one line in the implementation of tuple.

template <size_t k, class... Ts>
typename std::enable_if<
    k == 0, typename elem_type_holder<0, tuple<Ts...>>::type&>::type
get(tuple<Ts...>& t) {
  return t.tail;
}

template <size_t k, class T, class... Ts>
typename std::enable_if<
    k != 0, typename elem_type_holder<k, tuple<T, Ts...>>::type&>::type
get(tuple<T, Ts...>& t) {
  tuple<Ts...>& base = t;
  return get<k - 1>(base);
}

What does tuple<Ts...>& base = t; mean? I think type of t is tuple<T, Ts...>& which cannot be converted to tuple<Ts...>&.