Write fib(n) returning the nth Fibonacci number, with fib(0) = 0 and fib(1) = 1.
The plain recursive version recomputes the same values exponentially often - fib(35) alone is around 30 million calls. Remember what you have already worked out.
fib(70) must return promptly. If your solution hangs, that is the exercise telling you it is not memoised.