Select 3 points, measures the angle at the 2nd point.

It would be particularly useful on hillside vessel attachments, such as clip pairs.

cos(theta) = (|x| dot |y|)/(||x|| * ||y||)

|x| dot |y| -> xi*yi+xj+yj+xk*yk

||x|| = SRSS(xi, xj, xk)