Imagine an object, for example a Cube.
Now Imagine the Camera.
Imagine that, whatever the position of the camera is, you always want (a certain face of) the Cube to face the camera.
This means that the normal of the Cube's pointing-towards-the-camera-face will change.
Now at the same time I would like the Cube to rotate around its (Z-)normal ... such that it is spinning around at a constant speed (the input can be the animation's timeframe).
I have been trying for days but I can't make it work.
Also note that non-animation-nodes dynamic ways to let an object face the camera (e.g. Object constraint and Track to) do not seem to make it easy to let the object, at the same time spin around itself ...