xt">(quickselect arr k &amp;opt left right)

Rearrange items in `arr` so that all items in `[left, k]` range
are the smallest.
The `k`-th element will have the `(k - left + 1)`-th smallest value
in `[left, right]`.
Mutates `arr`.</pre><a href="https://janetdocs.org/math/quickselect">Community Examples</a></div><hr><div class="binding"><span id="math/relative-err" class="binding-sym">math/relative-err</span> <span class="binding-type">function</span> <span class="binding-type"><a href="https://github.com/jan