public XmlGraphNode CreateNode(string id = null)
        {
            var node = new XmlGraphNode(this, id ?? Guid.NewGuid().ToString());

            _nodes.Add(node.Id, node);
            return(node);
        }
 public XmlGraphConnection(XmlGraphNode from, XmlGraphNode to)
 {
     this.From = from;
     this.To   = to;
 }