theorem
  seq1 is summable & seq2 is summable implies seq1 - seq2 is summable &
  Sum(seq1 - seq2) = Sum(seq1) - Sum(seq2)
proof
  assume seq1 is summable & seq2 is summable;
  then
A1: Partial_Sums(seq1) is convergent & Partial_Sums(seq2) is convergent;
  then Partial_Sums(seq1) - Partial_Sums(seq2) is convergent by CLVECT_2:4;
  then Partial_Sums(seq1 - seq2) is convergent by Th2;
  hence seq1 - seq2 is summable;
  thus Sum(seq1 - seq2) = lim (Partial_Sums(seq1) - Partial_Sums(seq2)) by Th2
    .= Sum(seq1) - Sum(seq2) by A1,CLVECT_2:14;
end;
