Description:
Returns the angle of length-N vectors V and W in N-dimensional space.
This is a value in [0, pi/2].
If one or both of the vectors have magnitude zero, then the angle
between them is not defined. But this function adopts a pragmatic
strategy: if both magnitudes are zero, then it returns 0; if one
magnitude is zero and the other is nonzero, then it returns pi/2.
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