code: function notifyStepProgress(element, progress) {
      const index = getIndex(element);
      if (progress !== undefined) stepStates[index].progress = progress;
      const resp = { element, index, progress: stepStates[index].progress };
  
      if (stepStates[index].state === 'enter') cb.stepProgress(resp);
    }
docs: * NOTIFY CALLBACKS **