class Extensible is the parent class of many classes such as User and Channel. class Extensible implements a system which allows modules to 'extend' the class by attaching data within a map associated with the object. In this way modules can store their own custom information within user objects, channel objects and server objects, without breaking other modules (this is more sensible than using a flags variable, and each module defining bits within the flag as 'theirs' as it is less prone to conflict and supports arbitary data storage).