WorkflowEventKindRefresh indicates client should re-fetch current instance of workflow for updated state.

ErrEOF means end of workflow.

ErrIncompatibleInput means the input reactor cannot react to the input.

ErrNoChange means the input does not cause the workflow to change.

ErrSameNode means the input is reacted to, but no node is produced.

ErrUpdateNode means the input is reacted to, but instead of producing a new node to be appended, the returned node should replace the node.

Intent can optionally implement CookieGetter.

NodeSimple can optionally implement CookieGetter.