
theorem Th3:
  for L being non empty RelStr holds id L is infs-preserving
proof
  let L be non empty RelStr;
  let X be Subset of L;
  set f = id L;
  assume ex_inf_of X, L;
  hence ex_inf_of f.:X, L by FUNCT_1:92;
  f.:X = X by FUNCT_1:92;
  hence thesis;
end;
