The loop over grid points

    int i,j;
    for (i = 0; i < nx; i++) {
      for (j = 0; j < ny; j++) {
	PWOTuple arglist(Py_BuildValue("(dd)", xcoor(i), ycoor(j)));
	PWONumber result(func1.call(arglist));
	a(i,j) = double(result);
      }
    }