The function swap_2 is used to swap the arguments
of a functor. The result is a functor f' that calls the original
functor f with the second and third argument exchanged, that is
f'(x,y,z,...)= f(x,z,y,...).
returns a functor equivalent to f, but
where the second and third argument are exchanged. Requirement: F is a model for
AdaptableFunctor with arity 3 ar 5.
ar