procedure Insert(data: integer; var head: List);
var
  p: List;
begin
  new(p);
  p^.elem := data;
  if head = nil then
  begin
    head := p;
    p^.next := nil;
    p^.pred := nil;
  end
  else if head^.next = nil then
  begin
    head^.next:=p;
    p^.pred := head;
    p^.next := nil;
  end
  else
    Insert(data, head^.next);
end;