let
  f := map n to if n = 0 then 1 else n * f(n - 1); 
in
  let
    f := map n,m,k to if (n <= 0 & n >= 0)
                  | (n < 0 & n > 0 & n != 0) then number?
                                           else m / f(k + 1);
  in
     let x:=3;
         y:=4;
         z:=cons?(function?(x ^ ~y), cons(-arity(x)));
     in
        let x=3;
            y:=4;
            z:=g();
        in
            (g(x,y,z))(null?(true),list?(false),first(null))