Function read_group_order_response 

fn read_group_order_response(
    buf: &mut impl Buf,
) -> Result<GroupOrder, CodecError>

Read a Group Order from a message that must name a real order.

SUBSCRIBE_OK, PUBLISH, PUBLISH_OK and FETCH_OK, and TRACK_STATUS_OK, whose format “is identical to the SUBSCRIBE_OK message” each say “Values of 0x0 and those larger than 0x2 are a protocol error”: a responder reports the order it settled on, so deferring to the publisher is not an answer it can give. SUBSCRIBE, TRACK_STATUS and FETCH are the requests, and there 0x0 is exactly how a subscriber says it has no preference — “the original publisher’s Group Order SHOULD be used”. The two readers cannot be merged without either refusing traffic the requests permit or accepting a reply that tells the subscriber nothing.