const double& matrix::operator()(size_t x, size_t y) const { if (!ptr_) throw std::runtime_error("empty gsl::matrix"); return *gsl_matrix_const_ptr(ptr_, x, y); }
/** Get element (i,j) */ double matrix<double>::operator()(const size_t& i, const size_t& j) const { const double *x = gsl_matrix_const_ptr(_matrix, i, j); return *x; }