Method operator-

inst/include/Rcpp/vector/MatrixRow.h:257–257

        const_iterator operator+(difference_type n) const { return iterator( row, index + n ) ; }
        const_iterator operator-(difference_type n) const { return iterator( row, index - n ) ; }
        difference_type operator-(const const_iterator& other) const { return index - other.index ; }

        const_iterator& operator+=(difference_type n) { index += n ; return *this ;}