Computes the angle between two vectors.
v are vectors of the same dimension, then
angle(u, v) is the angle between them.
For real vectors,
angle(u, v) = arccos((u | v) / [norm(u)⋅norm(v)])
For complex vectors,
angle(u, v) = arccos(Re(u | v) / [norm(u)⋅norm(v)]).
∠ is a synonym of
∠(❨2, 1, 0, 2❩, ❨−1, 2, 1, 0❩)