result out(
stateT& state,
const internT* from, const internT* from_end, const internT*& from_next,
externT* to, externT* to_end, externT*& to_next) const;
result unshift(stateT& state, externT* to, externT* to_end, externT*& to_next) const;
result in(
stateT& state,
const externT* from, const externT* from_end, const externT*& from_next,
internT* to, internT* to_end, internT*& to_next) const;
int encoding() const noexcept;
bool always_noconv() const noexcept;
int length(stateT& state, const externT* from, const externT* from_end, size_t max) const;
int max_length() const noexcept;