he Class system in [Mootools](http://mootools.net) a exceptionally powerful for the little amount of code that it is comprised of. A large part of that power comes from the Mutator system. A Mutator can be thought of a lot like a plug-in. However unlike most plug-ins that are executed at run time, Mootools Mutators are executed at the time a class is [actually defined](http://github.com/mootools/mootools-core/blob/master/Source/Class/Class.js#L76) altering the end result of the class

A Mutator is nothing more than a JavaScript function that has access to the internals of the class it is attached to. These functions are stored in a Mutators namespace in the Mootools library.