---> <Apt2> at src/App.vue
       <Root>
warn @ vue.esm.js?efeb:476
vue.esm.js?efeb:476 [Vue warn]: Property or method "seen" is not defined on the instance but referenced during render. Make sure to declare reactive data properties in the data option.

found in

---> <Apt2> at src/App.vue
       <Root>


App.vue------------------------------------------------------------------------

<template>
 <div id="app-2">
  <span v-if="seen">Now you see me</span>
</div>
</template>


<script>
export default {
  name: 'apt-2',
  data () {
     seen: true
  }
}
</script>