Function sp_finality_grandpa::sign_message

pub fn sign_message<H, N>(
    keystore: SyncCryptoStorePtr, 
    message: Message<H, N>, 
    public: AuthorityId, 
    round: RoundNumber, 
    set_id: SetId
) -> Option<SignedMessage<H, N, AuthoritySignature, AuthorityId>> where
    H: Encode,
    N: Encode, 

Localizes the message to the given set and round and signs the payload.