code: function moveVirtual(tag, src, target, len) {
    var el = tag._root, sib, i = 0
    for (; i < len; i++) {
      sib = el.nextSibling
      src.insertBefore(el, target._root)
      el = sib
    }
  }
docs: Move virtual tag and all child nodes
  @param { Tag } tag - first child reference used to start move
  @param { Node } src  - the node that will do the inserting
  @param { Tag } target - insert before this tag's first child
  @param { Number } len - how many child nodes to move