Trailing path - Drawing

[Kirl]

I want to create a trailing path (for a moving ball) with the drawing api.

Is there an easy way a line (drawn with the drawing api) can erase its tail, or do I have to think of some sort of clever scheme to do this? The ball may move in a curved path, so I can't calculate two points to make the line.

Also, how can I limmit drawing commands to any particular (dynamically created) layer?

Thanks