const ex & operator() (unsigned ro, unsigned co) const;
ex & operator() (unsigned ro, unsigned co);
matrix & set(unsigned ro, unsigned co, const ex & value) { (*this)(ro, co) = value; return *this; }
const ex & operator() (unsigned ro, unsigned co) const;
ex & operator() (unsigned ro, unsigned co);
matrix & set(unsigned ro, unsigned co, const ex & value) { (*this)(ro, co) = value; return *this; }