Usefulness of the "not in" operator

[Roy Smith]

In article <87ehyn8xlp.fsf at dpt-info.u-strasbg.fr>,
 Alain Ketterlin <alain at dpt-info.u-strasbg.fr> wrote:

> Sure, but note that you can also reformulate != using not and ==, <
> using not and >=, etc. Operators like "not in" and "is not" should
> really be considered single tokens, even though they seem to use "not".
> And I think they are really convenient.

If you want to take it one step further, all the boolean operators can 
be derived from nand (the dualists would insist on using nor).