theorem
  seq1 - (seq2 + seq3) = seq1 - seq2 - seq3
proof
  thus seq1-(seq2+seq3)=seq1+(-1)*(seq2+seq3) by Th11
    .=seq1+((-1)*seq2+(-1)*seq3) by Th12
    .=seq1+(-seq2+(-1)*seq3) by Th11
    .=seq1+(-seq2+-seq3) by Th11
    .=seq1-seq2-seq3 by Th10;
end;
