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Kernel::Point_2
        
         | 
        
    fo.operator() (  Kernel::Line_2 l,  int i)
        
         | 
 |  | 
    returns an arbitrary point on l. It holds 
        point(i) == point(j), iff i==j.
        Furthermore,  is directed from point(i)
        to point(j), for all i < j.
         | 
        
  | 
        | 
        
    
Kernel::Point_2
        
         | 
        
    fo.operator() (  Kernel::Ray_2 r,  int i)
        
         | 
 |  | 
    returns a point on r. point(0) is the source,
        point(i), with i>0, is different from the 
        source. 
         | 
        
  | 
        | 
        
    
Kernel::Point_2
        
         | 
        
    fo.operator() (  Kernel::Segment_2 s,  int i)
        
         | 
 |  | 
    returns source or target   of s:   point(0) returns
        the source of s, point(1) returns the target of s.
        The parameter i is taken modulo 2, which gives 
        easy access to the other end point.
         |