code: function walk (newNode, oldNode) {
    // if (DEBUG) {
    //   console.log(
    //   'walk\nold\n  %s\nnew\n  %s',
    //   oldNode && oldNode.outerHTML,
    //   newNode && newNode.outerHTML
    // )
    // }
    if (!oldNode) {
      return newNode
    } else if (!newNode) {
      return null
    } else if (newNode.isSameNode && newNode.isSameNode(oldNode)) {
      return oldNode
    } else if (newNode.tagName !== oldNode.tagName) {
      return newNode
    } else {
      _$morph_11(newNode, oldNode)
      updateChildren(newNode, oldNode)
      return oldNode
    }
  }
docs: Walk and morph a dom tree