The "Trail Mesh" example code needs modification + I need some clarification

In the documentation for Trail Mesh Trail Mesh | Babylon.js Documentation some example code is provided.

At the beginning, alpha is declared as const alpha = Math.PI;

The scene freezes up when I add this code:

observer = scene.onBeforeRenderObservable.add(animate);
    function animate() {
        alpha += Math.PI/120;
        cube.position.x = Math.sin(alpha) * 10;
        cube.position.y = Math.cos(alpha) * 10;
        cube.rotation.x = (Math.PI * alpha) / 2;
        cube.rotation.y = alpha;

yielding an error in the browser console about assignment to a constant variable. I believe alpha has to be declared as let alpha = ... for the code to work.

Additionally, I do not know why observer can work in this case, even though it has not been declared. If anyone can offer some insight, that would be very helpful.

Thanks for reporting, it should be updated soon.