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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