primitive SendRequestOK
"""
Request was serialized and sent successfully.
"""
primitive ConnectionClosed
"""
Connection is not open, or a send failed and the connection was closed.
"""
primitive ResponsePending
"""
A response to a previous request is still in progress.
"""
type SendRequestError is (ConnectionClosed | ResponsePending)
"""Error returned by `send_request()` when the request cannot be sent."""
type SendRequestResult is (SendRequestOK | SendRequestError)
"""Result of `send_request()`: either success or an error."""