public abstract void Remove(DrawingElement d);
public override void Remove(DrawingElement d) { elements.Remove(d); }
public abstract void Add(DrawingElement d);
public override void Add(DrawingElement d) { elements.Add(d); }
public abstract void Remove(DrawingElement d);
public override void Remove(DrawingElement d) { elements.Remove(d); }
public abstract void Add(DrawingElement d);
public override void Add(DrawingElement d) { elements.Add(d); }