Cmd accepts a Dispatch function and an Error handler.

Dispatch is a function in F# for natural usage.

The Program type that holds the configuration of the MVU app.

Sub is the set of subscriptions a program wants active for the current model. The engine starts and stops subscriptions to match this set on every model change; ids must be unique within the set.

SubId identifies one subscription inside a program. It is a list of strings so callers can scope child subscriptions with a prefix and embed dependency values (for example ["timer"; "100"]) that restart the subscription when they change.

Subscribe starts one subscription and returns the disposable that stops it.