 
 public enum GATTClientError: Error {
     
-    /// The internal operations queue is full and no more pending ATT operations can be queued.
-    ///
     /// The request was not successfully sent because the underlying transmit queue is full.
     case queueFull
     

        
    }
    
    @inline(__always)
    private func send <Request: ATTProtocolDataUnit, Response: ATTProtocolDataUnit> (_ request: Request, response: @escaping (ATTResponse<Response>) -> ()) -> UInt? {
        
        log?("Request: \(request)")

            sendOperationID = send(pdu) { [unowned self] in self.readByGroupType($0, operation: operation) }
        }
        
        /// immediately call completion with error
        guard sendOperationID != nil else {
            completion(.error(.queueFull))
            return
        }
    }
    
