code: function (arg) {
    var result
    switch (arg) {
      case 'on':
        result = arg
        break
      case 'once':
        result = arg
        break
      case 'class':
        result = arg
        break
      case 'style':
        result = arg
        break
      case 'enter':
        result = arg
        break
      case 'leave':
        result = arg
        break
      case 'animate':
        result = arg
        break
      case 'parent':
        result = arg
        break
      default:
        console.warn('[in-view] argument ${arg} undefined')
    }
    return result
  }
docs: (arg → string)
  return string
  - check and get argument