applyAtPath(path, callback) {
    let diff = {objectId: '_root', type: 'map', props: {}}
    callback(this.getSubpatch(diff, path))
    this.applyPatch(diff, this.cache._root, this.updated)
  }
Constructs a new patch, calls `callback` with the subpatch at the location `path`,
and then immediately applies the patch to the document.