Quoting Paolo Capriotti <p.capriotti / gmail.com>:

> I propose the following for the next week quiz:
>
> Write a ruby program that takes a sentence as its input an
> produces an output that autoreferentially describes the letters
> found in the sentence _and_ the description itself.

Interesting.  This is basically equivalent to writing a function
which takes a message and produces a combined message + header,
where the header includes a checksum/digest of the combined message
+ header.

I don't believe that's possible in general, but it may be in
specific cases, depending on the properties of the particular
checksumming algorithm chosen.

-mental