Usefulness of the "not in" operator

[Ian Kelly]

On Mon, Oct 10, 2011 at 11:33 AM, Chris Angelico <rosuav at gmail.com> wrote:
> On Tue, Oct 11, 2011 at 4:29 AM, Nobody <nobody at nowhere.com> wrote:
>>
>> The Church numeral for N is a function of two arguments which applies its
>> first argument N times to its second, i.e. (f^N)(x) = f(f(...(f(x))...)).
>>
>
> Thanks - nice clear explanation. Appreciated. For an encore, can you
> give an example of where this is actually useful? It seems a pretty
> narrow utility.

It's useful for writing mathematical theorems about computability with
regard to the natural numbers using lambda calculus.