MixinThe base class for mixins — method bundles a class pulls in with .mix:. Define one by subclassing (Mixin <- MyMixin <- { ... }); mixins are never instantiated.
Requirements hook: called with the host class at the end of a host's definition body after it .mix:es this mixin (so required methods may be defined below the .mix: line). Throw to reject the host — it is then never registered. This default accepts every host.