Usefulness of the "not in" operator

[Chris Angelico]

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.

ChrisA