Hi,
I just noted that the official description of the Wikibase data model at
https://www.mediawiki.org/wiki/Wikibase/DataModel
and the description of JSON serialization lack a description of Lexemes, Forms, and Senses. The documentation of Wikibase RDF has been updated but the underlying data model description seems outdated.
At least JSON specification is part of Wikibase source code:
https://phabricator.wikimedia.org/diffusion/EWBA/history/master/docs/json.wi...
A more formal document (e.g. JSON Schema) may help to detect when implementation and documentatation get out of sync.
Jakob