post
Subscribes the consumers to events to receive events resulting from the command as they happen, then sends the command to the server.
The returned Subscription instances should be canceled after the requesting code receives expected events, or after a reasonable timeout.
The method returns subscriptions to events that the handling of the command may produce. A command may not be accepted for processing by the server, e.g. because of a validation error. In such a case, the method would report the error to the configured error handler, and return an empty set.
Return
subscription to the observed events if the command was successfully posted, or an empty set if posting caused an error