Write tests for this

Write {{framework}} tests for the code below.

Before writing them, list the cases where this code could plausibly be wrong —
boundaries, empty and single-element inputs, ordering, concurrent access, error
paths. Then write a test per case, named after the behaviour it protects rather
than the method it calls.

Skip tests that only restate the implementation. If a case cannot be tested
without changing the code, say which change would make it testable.

{{code}}

What to fill in

{{framework}}

Test framework — e.g. XCTest

{{code}}

Code under test — e.g. Paste the function or type