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

theorem Th28:
  Fib (n + 2) = Fib (n + 3) - Fib (n + 1)
proof
  Fib (n + 3) = Fib (n + 1 + 1 + 1) .= Fib (n + 1) + Fib (n + 2) by PRE_FF:1;
  hence thesis;
end;
