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I think I've worked out how to iron out most of the kinks that were caused by this bug, and just need to implement it in the code. Along the plane with a normal in the direction from the last to the next point, The idea is to compare the actual rotation with an ideal rotation that minimises rotation from the previous point. This requires me to add in an additional pre-rotation argument at each point, which might be useful to others (it's a bit like the 'twist' argument in the current linear_extrude, but applied on a per-point basis).