When we wrote the book, the latest Kotlin version was 1.4. Kotlin 1.5 was released just as we went to print. At the time I'm writing this page, Kotlin 1.6 is imminent. The underlying design principles we describe in the book do not depend on the version of the language (or even on Kotlin itself). However, the details of some code examples and refactoring sequences will become outdated as the language and tooling evolves. Here we document those differences.