The server keeps the current revision number and a list of previous operations to transform incoming operations against.
|:: OTSystem doc op|
|=> ServerState doc op|
The latest operation that the client has received from the server when it sent the operation.
The operation received from the client.
|-> Either String (op, ServerState doc op)|
The operation to broadcast to all connected clients (except the client which has created the operation; that client must be sent an acknowledgement) and the new state (or an error).
Handles incoming operations.