- instances (verbs)
- objects (nouns)
- sentence = verb + nouns

=> separate documents from session information

generic data store, convention for how entries look like
distinguish records by type
records have metadata and optionally data

- create(metadata, data):
  - add new entry, assign ids
  - data='' indicates no data to store
  - bad design? (data OOB)

- add_version(tree_id, parent_id, metadata, data)
  - add new version to existing object

- change_metadata(tree_id, version_id, metadata)
  - change the metadata of an existing version

- restore(tree_id, parent_id, version_id, metadata, data)
  - add a new version with the given ids
  - there must be no existing entry with the same (tree_id, version_id)
  - if parent_id != '' there must be an existing entry (tree_id, parent_id)
  - if parent_id = '', there must be no existing entry with the same tree_id and no parent_id