A model fo of this type must provide:

fo ( SphericalKernel::Line_3 l ,
SphericalKernel::Circular_arc_point_3 p ,
SphericalKernel::Circular_arc_point_3 q )
Constructs the line segment supported by l, whose source is p and whose target is q.
Precondition: p and q lie on l and are different.

SphericalKernel::Line_arc_3 fo ( SphericalKernel::Segment_3 s )

SphericalKernel::Line_arc_3 fo ( SphericalKernel::Point_3 p , SphericalKernel::Point_3 q )