    const_iterator &operator+=(difference_type rhs) {
      if (charPtr_) {
        charPtr_ += rhs;
      } else {
        char16Ptr_ += rhs;
      }
      return *this;
    }
    const_iterator &operator+=(difference_type rhs) {
      if (charPtr_) {
        charPtr_ += rhs;
      } else {
        char16Ptr_ += rhs;
      }
      return *this;
    }