     sentinel.treeId = @treeIdCount++
     # save Index, so we can access it once we have the root element
     sentinel.treeIndex = @trees.length
+    sentinel.isSentinel = true
     @trees.push(sentinel)
     @activeTree = sentinel
     @activeNode = null

    # to create one.
    if @trees.length == 0
      @createNewTree()
    @activeTree = @trees[0]
    @activeNode = null
    @push()

  getTree : (id) ->

      @activeNode.remove(id)
      @lastActiveNodeId = @activeNode.id
    else
      # Root is deleted
      @deleteActiveTree()
    @push()

