The interface class Pm_segment_traits_2<Kernel> is a model of the
PlanarMapTraits_2 concept. It is provided as part of the package and
can be used as reference for resolving ambiguities.
It must be templated with a CGAL Kernel model that is templated in turn
by an exact number type, in order to avoid robustness problems,
although other number types could be used at the user's own risk.
For example, Cartesian<leda_rational>,
Homogeneous<leda_integer>,
leda_rat_kernel_traits.
