acet-geom-arc-bulge

(acet-geom-arc-bulge  center  p1  deltaang)

Returns the bulge value for specified difference angle deltaang. arguments center and p1 are unused

The bulge is the tangent of 1/4 of the included angle for an arc segment, made negative if the arc goes clockwise from the start point to the endpoint.
A bulge of 0 indicates a straight segment, and a bulge of 1 is a semicircle.