 
return type of the
functor.
 
 
defines the arity of the functor,
i.e., the number of arguments it takes. The class has to be a
specialization of CGAL::Arity_tag<int>, where the template
parameter corresponds to the arity of the functor, e.g.
CGAL::Arity_tag<2> for binary functors.


 
(as many arguments as defined by Arity) returns f(a1,...an). 