message: reactor.network.SocketOperation: Expose `AsioSocket` to subclasses.
diff: @@ -15,10 +15,13 @@ namespace elle
  /// Abstract specialization of Operation for Sockets.
  ///
  /// This implements Operation::_abort.
  - template <typename AsioSocket>
  + template <typename S>
  class SocketOperation
  : public Operation
  {
  + public:
  + using AsioSocket = S;
  +
  public:
  /// Create a SocketOperation.
  ///
  