/*! Create a vertex v that matches v2, which lies of the edge e1. */
virtual void create_vertex(Halfedge_const_handle e1, Vertex_const_handle v2,
Vertex_handle v) const
{ v->set_data(e1->data() + v2->data()); }
/*! Create a vertex v that matches v1, which lies of the edge e2. */
virtual void create_vertex(Vertex_const_handle v1, Halfedge_const_handle e2,
Vertex_handle v) const
{ v->set_data(v1->data() + e2->data()); }
/*! Create a vertex v that matches v1, contained in the face f2. */
virtual void create_vertex(Vertex_const_handle v1, Face_const_handle f2,
Vertex_handle v) const
{ v->set_data(v1->data() + f2->data());}
/*! Create a vertex v that matches v2, contained in the face f1. */
virtual void create_vertex(Face_const_handle f1, Vertex_const_handle v2,
Vertex_handle v) const
{ v->set_data(f1->data() + v2->data()); }