The Enumerable mixin provides collection classes with several
traversal and searching methods, and with the ability to sort. The class
must provide a method `each`, which yields successive members of the
collection. If {Enumerable#max}, {#min}, or {#sort} is used, the objects in
the collection must also implement a meaningful `<=>` operator, as
these methods rely on an ordering between members of the collection.
@ISO 15.3.2