The rocket card below is server-rendered (static HTML in this page), unlike in
tree-demo.html, where builtIns.tree creates it. Here,
builtIns.scope takes it over: it attaches the RocketManager named by the card's
itemscope, gives it the value named by its itemprop (vm.rocket), and binds the
card's itemprops and its button against the manager. The manager is the same kind as Tree's, ctx.rebind()
included. Discussion: Chats/AssignFrom/TODO/ReverseTree.md.