code: function addVirtual(tag, src, target) {
    var el = tag._root, sib
    tag._virts = []
    while (el) {
      sib = el.nextSibling
      if (target)
        src.insertBefore(el, target._root)
      else
        src.appendChild(el)
  
      tag._virts.push(el) // hold for unmounting
      el = sib
    }
  }
docs: Adds the elements for a virtual tag
  @param { Tag } tag - the tag whose root's children will be inserted or appended
  @param { Node } src - the node that will do the inserting or appending
  @param { Tag } target - only if inserting, insert before this tag's first child