About the book: year / New features of the FB2.1 format were used when preparing the document. In particular, a document has different accessibility depending on the user's access level (subscriber/stranger). Part of the document (most of it, in fact) becomes available only after payment by the user. This design method, in particular, will allow the author to independently place his book in the library, set its price and select a fragment for review. Documentation for new fb2.1 features is under development.