/// <summary>
        /// Gets the transactions those are unconfirmed using getrawmempool.
        /// This is efficient when many transaction ids are provided.
        /// </summary>
        public static async Task <IEnumerable <uint256> > GetUnconfirmedAsync(this RPCClient rpc, IEnumerable <uint256> transactionHashes)
        {
            uint256[] unconfirmedTransactionHashes = await rpc.GetRawMempoolAsync();

            // If there are common elements, then there's unconfirmed.
            return(transactionHashes.Intersect(unconfirmedTransactionHashes));
        }