code: function clipboardInit () {
        outline.clipboard = {}
        outline.clipboard.objects = []
        outline.clipboard.predicates = []
        outline.clipboard.all = []
      }
docs: /* clipboard principle: copy wildly, paste carefully
  ToDoS:
  1. register Subcollection?
  2. copy from more than one selectedTd: 1.sequece 2.collection
  3. make a clipboard class?