delete (nodeId) {
    var op = null
    var node = this.get(nodeId)
    if (node) {
      var nodeData = node.toJSON()
      op = ObjectOperation.Delete([nodeId], nodeData)
      this.apply(op)
    }
    return op
  }
Delete a node.

    @param {String} nodeId
    @returns {ObjectOperation} The applied operation.