reserve m, n for Nat;

theorem
  Moebius 2 = -1
proof
  Moebius 2 = (-1) |^ card support ppf 2 by Def3,INT_2:28
    .= (-1) |^ card {2} by Th8,INT_2:28
    .= (-1) |^ 1 by CARD_1:30;
  hence thesis;
end;
