theorem
  seq1 - (seq2 - seq3) = seq1 - seq2 + seq3
proof
  thus seq1-(seq2-seq3)=seq1+(-1)*(seq2-seq3) by Th11
    .=seq1+((-1)*seq2-((-1)*seq3)) by Th14
    .=seq1+(-seq2-((-1)*seq3)) by Th11
    .=seq1+(-seq2-(-seq3)) by Th11
    .=seq1+(-seq2+seq3) by Th17
    .=seq1-seq2+seq3 by Th10;
end;
