public override void Remove(DrawingElement drawingElement)
 {
     elements.Remove(drawingElement);
 }
 public override void Add(DrawingElement drawingElement)
 {
     elements.Add(drawingElement);
 }
 public override void Remove(DrawingElement drawingElement)
 {
     Console.WriteLine(
         "Cannot remove from a PrimitiveElement");
 }
 public override void Add(DrawingElement drawingElement)
 {
     Console.WriteLine(
         "Cannot add to a PrimitiveElement");
 }
 public abstract void Remove(DrawingElement drawingElement);
 public abstract void Add(DrawingElement drawingElement);