f = <span class="hljs-function">(<span class="hljs-params">                <span class="hljs-comment">// f is a recursive function taking:</span>
  [c,                <span class="hljs-comment">//   c   = next digit character</span>
      ...a],         <span class="hljs-comment">//   a[] = array of remaining digits</span>
  o = b,            <span class="hljs-comment">//   o   = output string</span>
  S = s        <span class="hljs-comment">//   S   = set of solutions</span>
</span>) =&gt;</span>  {}