A couple of commenters have mentioned this, but with his definition of fib that&#x27;s given right below in the &#x27;overview&#x27; section, the function works:<p><pre><code>  Fib(0) = 1
  Fib(1) = 1
  Fib(n) = Fib(n−1) + Fib(n−2)
</code></pre>
So n has a constraint of n &gt; 1, for which the function works.