public TreeElementViewModel AddChild(TreeElementViewModel child)
 {
     _children.Add(child);
     return child;
 }
 public void RemoveChild(TreeElementViewModel child)
 {
     _children.Remove(child);
 }
 public void RemoveChild(TreeElementViewModel child)
 {
     _children.Remove(child);
 }
 public TreeElementViewModel AddChild(TreeElementViewModel child)
 {
     _children.Add(child);
     return(child);
 }