private static void RemoveBlock()
        {
            DxfDocument dxf = new DxfDocument();
            Block block = new Block("MyBlock");

            Line line1 = new Line(new Vector3(-5, -5, 0), new Vector3(5, 5, 0));
            Line line2 = new Line(new Vector3(5, -5, 0), new Vector3(-5, 5, 0));
            block.Entities.Add(line1);
            block.Entities.Add(line2);

            Insert insert = new Insert(block);

            dxf.AddEntity(insert);

            bool ok;
            // line1 is used by block and cannot be removed (ok = false)
            ok = dxf.RemoveEntity(line1);
            // block is used by insert and cannot be removed (ok = false)
            ok = dxf.Blocks.Remove(block);
            // it is safe to remove insert, it doesn't belong to anybody (ok = true)
            ok = dxf.RemoveEntity(insert);
            // it is safe to remove block, it doesn't belong to anybody (ok = true)
            // at the same time, all entities that were part of the block have been also removed
            ok = dxf.Blocks.Remove(block);
            // obj is null the line1 does not exist in the document, the block was removed
            DxfObject obj = dxf.GetObjectByHandle(line1.Handle);

            Console.WriteLine("Press a key...");
            Console.ReadKey();

        }