Constant ACK_MORE_KEY 

pub const ACK_MORE_KEY: &str = "ack_more";

Metadata key on a delivery ACK that answers more than one frame: the ids after the one in ACK_FOR_KEY, comma-separated. Sent only to a peer that advertised ACK_COALESCED_V1, and never on an undecryptable answer. A peer that ignores it still settles the first id, and the rest through their resends’ duplicate re-acknowledgement.