CGAL::opposite

Oriented_side	opposite ( Oriented_side o)
		returns the opposite side (for example ON_POSITIVE_SIDE if o==ON_NEGATIVE_SIDE), or ON_ORIENTED_BOUNDARY if o==ON_ORIENTED_BOUNDARY.
Bounded_side	opposite ( Bounded_side o)
		returns the opposite side (for example BOUNDED_SIDE if o==UNBOUNDED_SIDE), or returns ON_BOUNDARY if o==ON_BOUNDARY.