namespace boost {
  namespace text {
    template<typename T, bool Const = std::is_const<T>::value> 
      struct optional_ref;

    template<> struct optional_ref<bool const, true>;
    template<> struct optional_ref<bool, false>;
    template<typename T> struct optional_ref<T, true>;

    template<typename Key, typename Value, typename Compare, 
             std::size_t KeySize> 
      struct trie;
    template<typename Key, typename Value> struct trie_element;
    struct trie_match_result;
  }
}