An object fo of this type must provide:
bool  fo ( SphericalKernel::Circular_arc_3 a )  
Tests whether the arc a is monotone, i.e. the intersection of
any meridian anchored at the poles of the context sphere used by the function SphericalKernel::is_theta_monotone_3_object
and the arc a is reduced to at most one point in general, and two points if a pole of that sphere is
an endpoint of a. Note that a bipolar circle has no such arcs.

SphericalKernel::MakeThetaMonotone_3