Locales and locale expressions in Isabelle/Isar

Locales provide a module system for the Isabelle proof assistant. Recently, locales have been ported to the new Isar format for structured proofs. At the same time, they have been extended by locale expressions, a language for composing locale specifications, and by structures, which provide syntax for algebraic structures. The present paper presents both and is suitable as a tutorial to locales in Isar, because it covers both basics and recent extensions, and contains many examples