docs: private void verifyFrameOpcode(WebSocketFrame frame) throws WebSocketException
      {
          switch (frame.getOpcode())
          {
              case CONTINUATION:
              case TEXT:
              case BINARY:
              case CLOSE:
              case PING:
              case PONG:
                  // Known opcode
                  return;
  
              default:
                  break;
          }
  
          // If extended use of web socket frames is allowed.
          if (mWebSocket.isExtended())
          {
              // Allow the unknown opcode.
              return;
          }
  
          // A frame has an unknown opcode.
          throw new WebSocketException(
              WebSocketError.UNKNOWN_OPCODE,
              "A frame has an unknown opcode: 0x" + Integer.toHexString(frame.getOpcode()));
      }