public override object Clone()
{
Vertex2D v1 = (Vertex2D)V1.Clone();
Vertex2D v2 = (Vertex2D)V2.Clone();
Vertex2D v3 = (Vertex2D)V3.Clone();
Triangle2D tr = new Triangle2D(v1, v2, v3);
tr.extID = extID;
tr.E1 = (Edge2D)E1.Clone();
tr.E2 = (Edge2D)E2.Clone();
tr.E3 = (Edge2D)E3.Clone();
return(tr);
}