It is recreated on every call to `fib`, but the `_fib`s get to reuse it. Using a cache makes `_fib` O(N) to compute, so it gives a performance benefit as long a

It is recreated on every call to `fib`, but the `_fib`s get to reuse it. Using a cache makes `_fib` O(N) to compute, so it gives a performance benefit as long as you aren't calling `fib(n)` with very small values of `n`.