theorem
  seq1-(seq2+seq3)=seq1-seq2-seq3
proof
  thus seq1-(seq2+seq3)=seq1+(-1r)(#)(seq2+seq3)
    .=seq1+((-1r)(#)seq2+(-1r)(#)seq3) by Th16
    .=seq1+(-seq2+(-1r)(#)seq3)
    .=seq1+(-seq2+-seq3)
    .=seq1-seq2-seq3 by Th7;
end;
