CGAL 4.12  2D Regularized Boolean SetOperations

Operations  
ArrDirectionalTraits::X_monotone_curve_2  operator() (ArrDirectionalTraits::X_monotone_curve_2 xc) 
accepts an \( x\)monotone curve xc and returns its opposite curve, namely a curve whose graph is the same as xc 's, and whose source and target are swapped with respect to xc 's source and target.  