A GM_Arc is defined by 3 points, and consists of the arc of the circle determined by the 3 points, starting at the first, passing through the second and terminating at the third. If the 3 points are co-linear, then the arc shall be a 3-point line string, and will not be able to return values for center, radius, start angle and end angle. <br/>NOTE In the model, a GM_Arc is a subclass of GM_ArcString, being a trivial arc string consisting of only one arc. This may be counter-intuitive in the sense that subclasses are often thought of as more complex than their superclass (with additional methods and attributes). A GM_Arc is simpler than a GM_ArcString in that it has less data, but it is more complex in that it can return geometric information such as "center", "start angle", and "end angle". This additional computational complexity forces the subclassing to be the way it is. In addition the "is type of" semantics works this way and not the other. <br/>In its simplest representation, the three points in the controlPoint sequence for an GM_Arc shall consist of, in order, the initial point on the arc, some point on the arc neither at the start or end, and the end point of the GM_Arc. <br/>GM_Arc::controlPoint : GM_PointArray = #lt; startPoint : GM_Position, <br/>             midPoint : GM_Position, <br/>             endPoint : GM_Position#gt;<br/><br/>If additional points are given, then all points must lie on the circle defined by any 3 non-colinear points in the control point array. All points shall lie on the same circle, and shall be given in the controlPoint array in the order in which they occur on the arc. <br/><br/>NOTE The use of the term "midPoint" for the center GM_Position of the controlPoint sequence is not meant to require that the GM_Position be the geometric midpoint of the arc. This is the best choice for this GM_Position from a computational stability perspective, but it is not absolutely necessary for the mathematics to work.<br/>