rem: int iter = solve(fine, norm);
add: int iter = mlt.solve(fine, norm);
context: int main(){  int I,J;  I = J = 40;  pair<double, double> x(-4,4);  pair<double, double> y(-4,4);  double arr_bound[] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};  vector<double> vec_bound(arr_bound, arr_bound + 20);  vec_bound.insert(vec_bound.end(),vec_bound.rbegin(),vec_bound.rend());  boundary bound_f(boundary::DIRICHLET, J, 0.0);  //boundary bound_f_odd(boundary::DIRICHLET, vec_bound.begin(), vec_bound.end());  boundary bound_f_odd(boundary::DIRICHLET, J, 1.0);  laplace fine(I, J, x, y, bound_f, bound_f, bound_f, bound_f_odd);  residual_norm norm(fine, EPS);  double norm_c0 = residual_norm::norm(fine);  int iter = solve(fine, norm);  double norm_c1 = residual_norm::norm(fine);  double ratio = norm_c1/norm_c0;  cout.precision(3);  cout << "# iter: " << iter << endl;  cout << "# ratio: " << ratio << endl;  cout << fine << endl;  return 0;}