code: function start (action, state) {
    initBuffer()
    if (__DEBUG__) {
      if (console.group) {
        displayBuffer.header = ['%credux-undo', 'font-style: italic', 'action', action.type]
        displayBuffer.action = colorFormat('action', colors.action, action)
        displayBuffer.prev = colorFormat('prev history', colors.prevState, state)
      } else {
        displayBuffer.header = ['redux-undo action', action.type]
        displayBuffer.action = ['action', action]
        displayBuffer.prev = ['prev history', state]
      }
    }
  }
docs: /* istanbul ignore next: debug messaging is not tested