functionWithUnderscores

const Z = f => (x => f(v => x(x)(v)))(x => f(v => x(x)(v)))

const Zero  = _ => __ => __ //2
const One   = ___ => ____ => ___(____) //4

const True = _____ => ______ => _____ //6
const False = _______ => ________ => ________ //8
const IsZero = _________ => _________(__________ => False)(True) //10


const Pair = ___________ => ____________ => _____________ => _____________(___________)(____________) // 13
const First = ______________ => ______________(True) //14
const Second = _______________ => _______________(False) //15


const Succ = ________________ => _________________ => __________________ => _________________(________________(_________________)(__________________)) //18
const Sum = ___________________ => ____________________ => _____________________ => ______________________ => ____________________(_____________________)(___________________(_____________________)(______________________)) //22

const Pred = _______________________ => First(
					_______________________(________________________ => Pair(
								Second(________________________))(
								Succ(Second(________________________))))
						(Pair(Zero)(Zero))) //24

const Mul = _________________________ => __________________________ => __________________________(Sum(_________________________))(Zero) //26

const If = ___________________________ => ____________________________ => _____________________________ => ___________________________(____________________________)(_____________________________) //29
const lif = ______________________________ => _______________________________ => ________________________________ => ______________________________(_______________________________)(________________________________)() //32

const _fact = _________________________________ => __________________________________ => lif(IsZero(__________________________________))(___________________________________ => One)(____________________________________ => Mul(__________________________________)(_________________________________(Pred(__________________________________)))) //36
const fact = Z(_fact)

const decodeNumber = n => n(i => i + 1)(0)
const encodeNumber = n => Array.from(Array(n).keys(), item => item + 1).reduce((p, n) => Succ(p), Zero);

const exec = f => n => decodeNumber(f(encodeNumber(n)));

//console.log(decodeNumber(fact(Five))); //120
//console.log(exec(fact)(5)); //120

exec(fact)(5)