Add an initial example of typical client code.

The idea is that this code can be greatly simplified by using one
of Scala's awesomest features: higher-order functions.
Use List.exists + higher-order functions to simplify client code.

Whereas the previous version of the code was just a poor man's
implementation of exists, we can instead lean on the libraries
and use a higher-order function to tersely express the search
condition (element (that is, underscore) is less than zero).