[RuleOn(typeof(ParentShapeContainsNestedChildShapes), FireTime = TimeToFire.TopLevelCommit)]
public class ShapeAddedToDiagramRule : AddRule
{
  private double offset = 0.25;
  private PointD location = new PointD(0.25, 0.25);

  public override void ElementAdded(ElementAddedEventArgs e)
  {
    Shape shape = null;
    ParentShapeContainsNestedChildShapes nestedLink = e.ModelElement as ParentShapeContainsNestedChildShapes;
    if (nestedLink != null)
    {
      shape = nestedLink.NestedChildShapes as Shape;
    }

    if (shape != null && shape.Diagram != null)
    {
      // Expand the shape and move it to its new position
      shape.IsExpanded = true;
      shape.Location = new PointD(location.X, location.Y + offset);

      // Adjust the height offset for the size of the shape
      // (I'm assuming that the DefaultContainerMargin
      // provides for a decent spacing between the shapes)
      offset += shape.Size.Height + shape.Diagram.DefaultContainerMargin.Height;
    }
  }
}