reserve x, y, z, E, E1, E2, E3 for set,
  sE for Subset-Family of E,
  f for Function of E, E,
  k, l, m, n for Nat;

theorem Th12:
  for n being odd Nat holds 1 <= n
proof
  let n be odd Nat;
  2 * 0 < n;
  then 0 + 1 <= n by NAT_1:13;
  hence thesis;
end;
