theorem Th27:
  {f,g}"" = {f",g"}
proof
  thus {f,g}"" = ({f}\/{g})"" by ENUMSET1:1
    .= ({f}"") \/ ({g}"") by Th23
    .= {f"} \/ ({g}"") by Th26
    .= ({f"}) \/ {g"} by Th26
    .= {f",g"} by ENUMSET1:1;
end;
