
theorem Th38:
  for k being Element of NAT, a, b being Element of SubstPoset (
  NAT, {k}) st a <= b & a = {{}} holds b = {{}}
proof
  let k be Element of NAT, a, b be Element of SubstPoset (NAT, {k});
  assume
A1: a <= b & a = {{}};
  Top SubstPoset (NAT, {k}) = {{}} by Th36;
  hence thesis by A1,WAYBEL15:3;
end;
