internal void ReplaceChild(NodeBase oldchild, NodeBase newchild)
{
int i = _children.IndexOf(oldchild);
_children.Insert(i, newchild);
_children.Remove(oldchild);
oldchild.Parent = null;
newchild.Parent = this;
}
internal void AddChild(NodeBase child)
{
_children.Add(child);
child.Parent = this;
}