acet-geom-arc-3p-d-angle

(acet-geom-arc-3p-d-angle  p1  p2  p3)

Returns the difference angle for a bulge between points p1 -> p2 -> p3 (this is, the included angle center-p1 and center-p3, with center defined by the 3 arcus/center points); the sign of the result specifies the direction.
angle is in radians.