reserve n for Nat,
  k for Integer;

theorem Th10:
  1 < n + 2
proof
  0 < n + 1 implies 1 < n + 2
  proof
    assume 0 < n + 1;
    0 + 1 = 1;
    hence thesis by XREAL_1:8;
  end;
  hence thesis;
end;
