Aren't symbols starting with underscore reserved !!!! [af]

As regards the Edge, Face, and Vertex members, what is the difference
between the antichain and the visibility complex. For example, for a
Vertex, is ccw_source_edge() the ccw source edge in the complex or only in
the antichain? -- Laurent Rineau, Michel Pocchiola, 2004/09/28

Remove opposite(Sign) -- Laurent Rineau, 2004/10/14