docs: function isEventValid (comp, type) {
    // if a component has aleary triggered 'appear' event, then
    // the 'appear' even† can't be triggered again utill the
    // 'disappear' event triggered.
    if (appearEvts.indexOf(type) <= -1) {
      return true
    }
    if (comp._appear === undefined && type === 'disappear') {
      return false
    }
    let res
    if (comp._appear === undefined && type === 'appear') {
      res = true
    }
    else {
      res = type !== comp._appear
    }
    res && (comp._appear = type)
    return res
  }