reserve x, X, Y for set;

theorem
  for L being non empty RelStr holds id L is monotone
proof
  let L be non empty RelStr;
  let l1,l2 be Element of L;
  assume l1 <= l2;
  then l1 <= (id L).l2 by FUNCT_1:18;
  hence thesis by FUNCT_1:18;
end;
