//  Example
    //
    // We use bilinear spline to interpolate f(x,y)=x^2+2*y^2 sampled
    // at (x,y) from [0.0, 0.5, 1.0,2.25] X [0.0, 1.0, 1.2].
    //
    real_1d_array x = "[0.0, 0.5, 1.0, 2.25]";
    real_1d_array y = "[0.0, 1.0, 1.2]";
    real_1d_array f =
        "[0.00,0.25,1.00,5.0625,2.00,2.25,3.00,7.0625,2.88,3.13,3.88,7.9425]";
    spline2dinterpolant ss;
    // build spline
    spline2dbuildbicubicv(x, 4, y, 3, f, 1, ss);

    double vx, vy, v, dx, dy, dxy;
    // calculate S(0.25,0.50)
    vx = 0.25;
    vy = 0.50;
    v = spline2dcalc(ss, vx, vy);
    printf("%.4f\n", double(v)); // EXPECTED: 0.5625

    // calculate derivatives
    spline2ddiff(ss, vx, vy, v, dx, dy, dxy);
    printf("%.4f\n", double(v)); // EXPECTED: 0.5625
    printf("%.4f\n", double(dx)); // EXPECTED: 0.5000
    printf("%.4f\n", double(dy)); // EXPECTED: 2.0000
       
    // calculate S(0.35,0.84)
    vx = 0.35;
    vy = 0.84;
    v = spline2dcalc(ss, vx, vy);
    printf("%.4f\n", double(v)); // EXPECTED: 1.5337

    // calculate derivatives
    spline2ddiff(ss, vx, vy, v, dx, dy, dxy);
    printf("%.4f\n", double(v)); // EXPECTED: 1.5337
    printf("%.4f\n", double(dx)); // EXPECTED: 0.7000
    printf("%.4f\n", double(dy)); // EXPECTED: 3.3600