Design patterns·Objects, messages and the principles
the class picks up three methods from somewhere that is not its parent, and finding where they came from takes a search.
Mixin
Also calledtrait, module include, mixin class
A bundle of behaviour designed to be mixed into a class rather than instantiated on its own, giving reuse across an inheritance tree without a common ancestor. It solves the problem that single inheritance only has one slot, and creates the problem that the set of methods on a class is no longer readable from its declaration. Languages disagree wildly on the details — traits, modules, protocol extensions — but the trade is the same everywhere.