Usefulness of the "not in" operator

[Alexander Kapps]

On 08.10.2011 18:08, Steven D'Aprano wrote:

> Let's define the boolean values and operators using just two functions:

[SNIP]

Have you just explained Church booleans in an understandable 
language? Awesome. I still have to chew on this, but I think this is 
the first time where I might understand it. Thanks!

Even if it's off-topic, could you add some similar explanations for 
Church numerals (maybe Lambda calculus it isn't too much?)