Later it was decided to change this so that diff_angle of
two zero vectors gives zero. Strictly speaking it should
perhaps be NaN, but it is already the case that norm of a
zero vector gives zero rather than NaN, so there is a precedent.
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This function is undefined in the case where either of the
vectors is (0,0,0), as is the case with vector(). So this
isn't really a "bug".
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Later it was decided to change this so that diff_angle of
two zero vectors gives zero. Strictly speaking it should
perhaps be NaN, but it is already the case that norm of a
zero vector gives zero rather than NaN, so there is a precedent.