function closeSection() {
    // A section is opened or opening. Try to cancel the opening transition first.
    this.openSectionTransition.cancel();

    // Run the actual close transition.
    this.closeSectionTransition =
        this.openSectionTransition.finished.then(function() {
          // The section already finished opening; close it.
          return new CloseSectionTransition();
        }, function() {
          // Cool, the opening was successfully canceled! Do nothing.
          return null;
        });
  }