### How to plot implicit3d plot with level sets in SageMath?

I want to plot a quadratic form with its level sets in SageMath. How to do that?

```
plot = Graphics()
plot+=plot3d(1/4*x^2+1/9*y^2,(x,-2,2),(y,-2,2))
for h in [0..5]:
plot+=contour_plot(f,(x,-4,4),(y,-4,4), fill=false, labels=true, contours=10,colorbar=true,cmap=matplotlib.cm.gist_rainbow).show(aspect_ratio=1)
show(plot)
```

How to achieve this?