nesting routing in the official website, the original text says
const router = new VueRouter({
  routes: [
    { path: "/user/:id", component: User,
      children: [
        {
          //  /user/:id/profile 
          // UserProfile  User  <router-view> 
          path: "profile",
          component: UserProfile
        },
        {
          //  /user/:id/posts 
          // UserPosts  User  <router-view> 
          path: "posts",
          component: UserPosts
        }
      ]
    }
  ]
})
 I want to ask why it matches  / user/:id/profile  rather than  / user/profile , because doesn"t  / user/profile  look more like a child route under user? And what is the function of the middle layer? Thank you 
