Equatable
Allows ruby objects to implement equality comparison and inspection methods.
By including this module, a class indicates that its instances have explicit
general contracts for `hash`, `==` and `eql?` methods. Specifically `eql?`
contract requires that it implements an equivalence relation. By default
each instance of the class is equal only to itself. This is a right
behaviour when you have distinct objects. However, it is the responsibility
of any class to clearly define their equality. Failure to do so may prevent
instances to behave as expected when for instance `Array#uniq` is invoked or
when they are used as `Hash` keys.
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