AOP is a mutator which let you write Aspect Oriented Classes in MooTools
Just include keeto.PatternMutator.js and kenta.AOP.js. look at the Spec directory to see a couple of example.
Included in this repository there is the Profiler example I wrote about in this post on my blog: http://mykenta.blogspot.it/2011/10/mootools-aop-and-how-to-use-aop-for.html
What kenta.AOP is not? It is not a complete AOP Framework. In particular kenta.AOP can't handle property access and it doesn't perform exception handling by design; it also overwrites all MooTools class methods, so I advise against using it for production-code :)