or you can build your own via the <code>reducer</code> function.</dd>
<dd><code>options</code> - Options map. Described below. May be null.</p>
<p>Options:</p>
<ul>
  <li><code>:map-initial-capacity</code> - initial hashmap capacity. Resizing hash-maps is expensive  so we would like to set this to something reasonable. Defaults to 100000.</li>
  <li><code>:index-filter</code> - A function that given a dataset produces a function from long index  to boolean. Only indexes for which the index-filter returns tr