theorem Th10:
  --{f,g} = {-f,-g}
proof
  thus --{f,g} = --({f}\/{g}) by ENUMSET1:1
    .= --{f} \/ --{g} by Th5
    .= {-f} \/ --{g} by Th9
    .= {-f} \/ {-g} by Th9
    .= {-f,-g} by ENUMSET1:1;
end;
