:: 2nd LISP Quiz at #lisp ::

> Problem:

Let's expand the official solution to the 1st LISP Quiz
(https://0x0.st/H7s4.txt), so that the `add` function can also deal
with negative numbers (one or both).

Don't use the + , the - , the * , the / , and the % mathematical
operations (1+ and 1- operators are allowed, which adds and subtracts
by one).

Don't create other auxiliary functions.

Maintain the recursiveness of the `add` function.

> Answer format:

Post your answer as a public pastebin-like link at this channel (
#lisp ). Test the result in Common LISP.

> Prize:

The winner will take home a good amount of coins. The amount of prize
coins is at the topic of the #lisp IRC channel. The prize goes to the
first person that submits a correct answer.

> Disclaimer:

I, danisanti, am prohibited to participate in this LISP Quiz. I only
manage the LISP Quiz.

> Final words:

Happy hacking!