theorem
  A+ ^^ (A?) = A? ^^ (A+)
proof
  thus A+ ^^ (A?) = A+ ^^ (A |^ (0, 1)) by FLANG_2:79
    .= A |^ (0, 1) ^^ (A+) by Th72
    .= A? ^^ (A+) by FLANG_2:79;
end;
