The class Triangulation_hierarchy_vertex_base_2<Vb> is designed to be used as a vertex base class of a triangulation plugged into a Triangulation_hierarchy_2<Tr>.
It is a model of the concept TriangulationHierarchyVertexBase_2 which refines the concept TriangulationVertexBase_2.
This class is templated by a parameter Vb which is to be instantiated by a model of the concept TriangulationVertexBase_2. The class Triangulation_hierarchy_vertex_base_2<Vb> inherits from the class Vb. This design allows to use either the default vertex base class or a user customized vertex base with additional functionalities.