theorem Th72:
  (A |^ (m, n)) ^^ (A+) = A+ ^^ (A |^ (m, n))
proof
  thus (A |^ (m, n)) ^^ (A+) = (A |^ (m, n)) ^^ (A |^.. 1) by Th50
    .= (A |^.. 1) ^^ (A |^ (m, n)) by Th25
    .= A+ ^^ (A |^ (m, n)) by Th50;
end;
