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[Sterpa <>]

I write a code to extract the k-level of an arrangement in the plane. This is 
a
simple code to know the level of a given halfedge, but I got an error trying 
to
access e.curve().line(), am I doing something wrong?

int getLevel(Halfedge e, vector<Line_2> lines) {

        int level = 0;

        NT x;
        bool source_finite, target_finite;

        if(e.direction() == CGAL::ARR_RIGHT_TO_LEFT) e = *e.twin();
        source_finite = !e.source()->is_at_open_boundary();
        target_finite = !e.target()->is_at_open_boundary();
        if(source_finite) {
                if(target_finite) x = CGAL::midpoint(e.source()->point(),
e.target()->point()).x();     
                else x = e.source()->point().x() + 1;                         
 
                        

        }

        else if(target_finite) x = e.target()->point().x() - 1;               
 
                        
        else x = 0;                                                           
 
                        
        Line_2 l = Line_2(-1, 0, x);
        Point_2 p = intersection(e.curve().line(), l);

        for(unsigned i = 0; i < lines.size(); i++) {

                Point_2 tmp = intersection(l, lines[i]);

                if(p.y() > tmp.y()) level++;

        }

        return level;

}

terminate called after throwing an instance of 'CGAL::Precondition_exception'
  what():  CGAL ERROR: precondition violation!
Expr: is_line()
File: /usr/local/include/CGAL/Arr_linear_traits_2.h
Line: 1700

I really need to fix this in short time, so thanks in advance to anyone. Best
regards