theorem
  R1 - R2 - R3 = R1 - (R2 + R3)
proof
  thus R1 - R2 - R3 = R1 - R2 + - R3 by FINSEQOP:84
    .= R1 + - R2 + - R3 by FINSEQOP:84
    .= R1 + (- R2 + - R3) by FINSEQOP:28
    .= R1 + -(R2 + R3) by Th31
    .= R1 - (R2 + R3) by FINSEQOP:84;
end;
