public static Task <Bill> GetBillAsync(this FreeAgentClient client, Uri url)
        {
            var id = url.GetId();

            return(client.GetBillAsync(id));
        }
        public static Task <Bill> GetBillAsync(this FreeAgentClient client, Bill bill)
        {
            var id = client.ExtractId(bill);

            return(client.GetBillAsync(id));
        }