The model of a document: how its operations apply. Each binding brings one (Kotoba's text, Hakushi's graph, a map for shared assigns), and the authority (Issho.Authority) runs it.

An engine is pure: no I/O, no clock, no randomness. The same operations on the same state give the same state on every node, in a test, and in the browser's twin of the engine. That is also what lets an engine move to a NIF or to WebAssembly unchanged.

Its state is a map of strings, numbers, booleans, nil, lists and maps with string keys (JSON) when browsers hold the document (they receive it, and run the engine's twin) or the store saves JSON. A document only LiveViews and agents hold, kept by no store or a store of terms, can be any Erlang term: a struct of your own.

Everything outside the engine treats operations as opaque: the authority, the store, receipts, the journal and the offline queue see only a transaction's id, its base revision and its author.

Callbacks

• name/0 and version/0 tag every stored state, so that a document can move to another engine, or a new version, on its own schedule.
• init/2 builds a state from the initial content your policy gives (Issho.Policy.document/2).
• apply/3 applies one transaction's operations, all or none.
• validate/2 checks a state and gives its content: what your app publishes and the store keeps beside the state (a rendered HTML, a graph). Without it, the content is the state.
• snapshot/1 is what a browser receives when it opens the document. Without it, the state.