Method either_iterator_impl

include/boost/parser/replace.hpp:113–113

                  concat_reference_t<V1, V2>>
        {
            constexpr either_iterator_impl() = default;
            constexpr either_iterator_impl(iterator_t<V1> it) : it_(it) {}
            template<typename V = V2>
            constexpr either_iterator_impl(iterator_t<V> it) : it_(it)