theorem
  seq1 - seq2 = seq1 + -seq2
proof
  let n be Element of NAT;
  thus (seq1 - seq2).n = seq1.n - seq2.n by NORMSP_1:def 3
    .= seq1.n + (-seq2).n by Th44
    .= (seq1 + -seq2).n by NORMSP_1:def 2;
end;
