theorem Th12:
  (A |^ m) ^^ (A |^ n) = (A |^ n) ^^ (A |^ m)
proof
  thus (A |^ m) ^^ (A |^ n) = A |^ (m + n) by FLANG_1:33
    .= (A |^ n) ^^ (A |^ m) by FLANG_1:33;
end;
