reserve E, x, y, X for set;
reserve A, B, C for Subset of E^omega;
reserve a, b for Element of E^omega;
reserve i, k, l, kl, m, n, mn for Nat;

theorem Th28:
  A |^ (n, n + 1) = A |^ n \/ A |^ (n + 1)
proof
  thus A |^ (n, n + 1) = A |^ (n, n) \/ A |^ (n + 1) by Th26,NAT_1:11
    .= A |^ n \/ A |^ (n + 1) by Th22;
end;
