let f0  = fun x -> (x, x) in
        let f1  = fun y -> f0(f0 y) in
        let f2  = fun y -> f1(f1 y) in
        let f3  = fun y -> f2(f2 y) in
        let f4  = fun y -> f3(f3 y) in
        let f5  = fun y -> f4(f4 y) in
        f5 (fun z -> z)