To manufacture a system with similar effects as an existing system, but somehow yours is better internally or comes about in a better fashion, I don't know. I cry foul on the very notion of prior art really.
To the tigers in the zoo, Madeline was like "POO POO".
It seems like the inverse operation doesn't hold up.
Engineer 1 invents a broadcast light source, the camp light, patents it and who would want to be walking around with a candle on a plate eh, eh? This is easier than moving your smoking coal fire.
Engineer 2 in the other valley lights up one of his hanging art pieces against the rock face and finds it lights up the whole area for an hour.
Engineer 1 sues engineer 2 for infringing on prior art of broadcasting light, and has all these people backing him up since the camp light sales just went way down who want their money.
That's how I see it and that's why I just duff around and put stuff in the public domain explicitly if anything, generally.