public class ListCell
  {
  private Object Datum;
  private ListCell Link;
  public ListCell(Object head, ListCell next_cell)
    {
    Datum = head; Link=next_cell; 
    }
  public Object GetDatum()
    {
    if (!(this==null))
      return this.Datum;
    else
      return null;
    }
  public ListCell GetLink()
    {
    if (!(this==null))
      return this.Link;
    else
      return null;
    }
  }