The class Straight_skeleton_builder_traits_2<Kernel> provides a model for the
StraightSkeletonBuilderTraits_2 concept which is the traits class
required by the Straight_skeleton_builder_2 algorithm class. The class
Straight_skeleton_builder_traits_2<Kernel> has one template argument: a 2D CGAL Kernel. This parameter must be a model for the Kernel concept, such as the Exact_predicates_inexact_constructions_kernel, which is the recommended one.
It is unspecified which subset of the kernel is usedd into the output sequence and the returned iterator will be equal to out.
For any given input polygon, it in this traits class (and by extension in the builder class). This is to avoid restricting the choices in the implementation.