Table 5:

Several levels of abstraction for a function f and their corresponding combinator structures. The combinator structures have no explicit variables (e.g. x, y, op). Note that if the constants or primitives +, 1, and 4 were defined with a Church encoding, they too would be combinators, permitting us to translate everything into pure S&K.

Function	Equivalent combinatory logic structure
(f) → (+ 1 4)	f := (+ 1 4)
(f x) → (+ x 1)	f := (S + (K 1))
(f x y) → (+ x y)	f := +
(f op x y) → (op x y)	f := (S K K)