http://en.wikipedia.org/wiki/Is-a http://en.wikipedia.org/wiki/Is-a versus http://en.wikipedia.org/wiki/Has-a http://en.wikipedia.org/wiki/Has-a. Usually the "

http://en.wikipedia.org/wiki/Is-a http://en.wikipedia.org/wiki/Is-a versus http://en.wikipedia.org/wiki/Has-a http://en.wikipedia.org/wiki/Has-a. Usually the "perfect" tree isn't really all that perfect.