Most fibers are partially determined, because they still retain a small mystical spark of self-determinism and so can summon up the willpower to do something unexpected, even if it jusr flapping or fraying or bending at the wrong moment. The more undetermined the fiber, the more random the outcome. Normal fiber is mostly determined, and can be used, for example, to weave cute sayings or pot holders with images of kittens embroidered on them. Try that with 1--% undetermined fibber and the kittens end up looking like gargoyles in a Klee nightmare.

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