Let zero x:x succ n:(s:(s select second) n) where select second x:y:y. Now let one succ zero two succ one

Let

zero Ξ λx:x
succ Ξ λn:(λs:(s select second) n)

where select second Ξ λx:λy:y. Now let

one Ξ succ zero
two Ξ succ one