reserve n, k, r, m, i, j for Nat;

theorem Th30:
  Fib (n) = Fib (n+2) - Fib (n+1)
proof
  Fib (n+2) - Fib (n+1) = Fib (n) + Fib (n+1) - Fib (n+1) by Th24
    .= Fib (n);
  hence thesis;
end;
