Figure 2.94 on page 322 illustrates a number of different variants of
the winged-edge data structure that enable determining all of the
edges that make up a face and that are incident at a vertex in both
clockwise and counterclockwise orders in time proportional to the
number of edges that make up the face or are incident at the vertex.
These combinations consist of four of the following eight possible
pointers for edge e:

1.  Next Clockwise Edge along Clockwise Face of e
2.  Next Clockwise Edge along Counterclockwise Face of e
3.  Next Counterclockwise Edge along Clockwise Face of e
4.  Next Counterclockwise Edge along Counterclockwise Face of e
5.  Next Clockwise Edge incident at starting vertex of e
6.  Next Clockwise Edge incident at ending vertex of e
7.  Next Counterclockwise incident at starting vertex of e
8.  Next Counterclockwise Edge incident at ending vertex of e

Which combinations enable determining all of the edges that make up a
face and that are incident at a vertex in both clockwise and
counterclockwise orders in time proportional to the number of edges that
make up the face or are incident at the vertex.