The Church numerals are a family of higher-order functionals that represent application of a function a given number of times. In other words:

0(f,x) == x

1(f,x) == f(x)

2(f,x) == f(f(x))

and so on.

The Church numerals figure heavily in combinator theory. The Combinator Engine includes an exposition of them in the SKI system.

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