code: public void subscribe(H.Session session, final String tag) {
          sessionRegistry().accept(session.id(), new $.Visitor<WebSocketConnection>() {
              @Override
              public void visit(WebSocketConnection connection) throws $.Break {
                  byTag.register(tag, connection);
              }
          });
      }
docs: Add tag to any websocket connection linked to the session specified
  @param session the session used to find websocket connections
  @param tag the tag to subscribe