    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;
    }