theorem
  (seq1 - seq2) ^\k = (seq1 ^\k) - (seq2 ^\k)
proof
  thus (seq1 - seq2) ^\k = (seq1 + (-seq2)) ^\k by CSSPACE:64
    .= (seq1 ^\k) + ((-seq2) ^\k) by Th86
    .= (seq1 ^\k) + -(seq2 ^\k) by Th87
    .= (seq1 ^\k) - (seq2 ^\k) by CSSPACE:64;
end;
