theorem Th20:
  --{a,b} = {-a,-b}
proof
  thus --{a,b} = --({a}\/{b}) by ENUMSET1:1
    .= --{a} \/ --{b} by Th15
    .= {-a} \/ --{b} by Th19
    .= {-a} \/ {-b} by Th19
    .= {-a,-b} by ENUMSET1:1;
end;
