theorem
  R1 - (R2 - R3) = R1 - R2 + R3
proof
  thus R1 - (R2 - R3) = R1 + - (R2 - R3) by FINSEQOP:84
    .= R1 + (- R2 + R3) by Th40
    .= R1 + - R2 + R3 by FINSEQOP:28
    .= R1 - R2 + R3 by FINSEQOP:84;
end;
