for (iter = 2; iter < NFRAMES; iter++)
    {
        boundary (u, N, iter, nsrc, src, ampl, timespacing);
        for (i = 1; i < N - 1; i++)
            for (j = 1; j < N - 1; j++)
                u[iter][i][j] += sqlambda * (   u[iter - 1][i + 1][j] +
                                                u[iter - 1][i - 1][j] +
                                                u[iter - 1][i][j + 1] +
                                                u[iter - 1][i][j - 1]
                                            ) + (2 - 4 * sqlambda) * u[iter - 1][i][j] - u[iter - 2][i][j];
    }