theorem Th44:
  R1 + (R2 - R3) = R1 + R2 - R3
proof
  thus R1 + (R2 - R3) = R1 + (R2 + - R3) by FINSEQOP:84
    .= R1 + R2 + - R3 by FINSEQOP:28
    .= R1 + R2 - R3 by FINSEQOP:84;
end;
