theorem Th24:
  r(#)(seq1-seq2)=r(#)seq1-r(#)seq2
proof
  thus r(#)(seq1-seq2)=r(#)seq1+r(#)(-seq2) by Th22
    .=r(#)seq1+((-1)*r)(#)seq2 by Th23
    .=r(#)seq1-(r(#)seq2) by Th23;
end;
