A model fo of this type must provide:

 
Constructs an arc from a full circle.  

 
Constructs the circular arc supported by c, whose source and target
are respectively p and q.

The circular arc constructed from a circle, a source, and a target, is defined as the set of points of the circle that lie between the source p1 and the target p2, when traversing the circle counterclockwise seen from the side of the plane of the circle pointed by its positive normal vectors.
In this definition, we say that a normal vector (a,b,c) is positive if (a,b,c)>(0,0,0) (i.e. $$(a>0)  (a==0) && (b>0)  (a==0)&&(b==0)&&(c>0)).
 
Constructs an arc that is supported by the circle of type
SphericalKernel::Circle_3 passing through the points p,
q and r. The source and target are respectively p
and r, when traversing the supporting circle in the
counterclockwise direction
seen from the side of the plane containing the circle pointed by its positive
normal vectors.
the circle.
Note that, depending on the orientation of the point triple
(p,q,r), q may not lie on the arc.
