 reserve n,s for Nat;

theorem
  Polygon (s,n) = (s - 3) * Triangle (n -' 1) + Triangle n
  proof
    (s - 3) * Triangle (n -' 1) + Triangle n =
      (s - 3) * (n * (n - 1) / 2) + Triangle n by Th21
      .= (s - 3) * (n * (n - 1) / 2) + n * (n+1) / 2 by Th19;
    hence thesis;
  end;
