theorem
  -(R1 - R2) = R2 - R1
proof
  thus -(R1 - R2) = -(R1 + -R2) by FINSEQOP:84
    .= -R1 + --R2 by Th31
    .= -R1 + R2 by Th28
    .= R2 + -R1 by FINSEQOP:33
    .= R2 - R1 by FINSEQOP:84;
end;
