
theorem Th21:
  for L be non empty RelStr holds [#]L is with_bottom with_top
proof
  let L be non empty RelStr;
  Bottom L in [#]L;
  hence [#]L is with_bottom by WAYBEL23:def 8;
  Top L in [#]L;
  hence thesis by WAYBEL23:def 9;
end;
