Sadly, this is probably the truth. (from http://www.quartertothree.com/fp/2016/01/29/the-witness-is-a-puzzle-wrapped-in-a-puzzle-inside-a-puzzle/#comment-2486166348 …)pic.twitter.com/5w3TaiCXHi
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Ah. Well, the immediate answer is that there are no actual collision planes in The Witness, they are all solids...
and there is actually no way to ever get inside them. But as to why nobody made a primitive that is one sided, that I can't say.
It does seem like it would be a useful primitive for some of the boundary regions where you're just saying "don't go past here".
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