Finish with a mixed review of the language behaviors used throughout the expert units. Trace an async continuation, distinguish regular and arrow receivers, unwrap a promise payload type, select a discriminated union member, transform a lazy sequence, clone supported built-in collections, and count guarded state changes.
These tasks require no new feature. They ask you to choose the right rule when several familiar rules appear together. Keep compile-time claims separate from runtime decisions: a useful union type can't tell whether a particular event is valid in the current state, and a generic clone return type can't prove that its mutable collections are detached. The hidden variants include empty strings, zero values, ignored events, and repeated inputs so a passing demonstration also handles realistic boundaries.