The class Straight_skeleton_2<Traits,Items,Alloc> provides a model for the
StraightSkeleton_2 concept which is the class
type used to represent a straight skeleton.
It inherits from HalfedgeDS_vector<Traits,Items,Alloc>
The only purpose of this class is to protect all the modifying operations in a HalfedgeDS. Normal users should not modify a straight skeleton. If an advanced user needs to get access to the modifying operations, it must call the required methods through the ::Base class.