🔒 We've studied `Equatable` and `Hashable`, their laws, and saw how value types as simple bags of data easily conform via "structural" equality. What about reference types? Reference types are an amalgamation of data _and_ behavior, and that data can be mutated in place at any time, so how can...

🔒 While the documentation for `Equatable` discusses the notions of "equivalence relation" _and_ "substitutability", there are conformances in the Standard Library that run afoul, but for pragmatic reasons. Let’s explore them and then dive deeper into a related protocol: `Hashable`.