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

theorem Th27:
  Fib (n + 5) = Fib (n + 3) + Fib (n + 4)
proof
  Fib (n + 5) = Fib (n + 3 + 1 + 1)
    .= Fib (n + 3) + Fib (n + 3 + 1) by PRE_FF:1;
  hence thesis;
end;
