theorem Th20:
  (seq1 - seq2) (#) seq3 = seq1 (#) seq3 - seq2 (#) seq3
proof
  thus (seq1-seq2)(#)seq3=seq1(#)seq3+(-seq2)(#)seq3 by Th15
    .=seq1(#)seq3-seq2(#)seq3 by Th18;
end;
