, a function applied to summarize the values from
the unused columns (i.e. columns not identified by <code>id_cols</code>,
<code>names_from</code>, or <code>values_from</code>).</p>
<p>The default drops all unused columns from the result.</p>
<p>This can be a named list if you want to apply different aggregations
to different unused columns.</p>
<p><code>id_cols</code> must be supplied for <code>unused_fn</code> to be useful, since otherwise
all unspecified columns will be considered <code>id_cols</code>.</