 <code>lhs</code> and <code>rhs</code>, keep the one from <code>lhs</code> in <code>union_prefer_left lhs rhs</code></p></div></div><div class="odoc-spec"><div class="spec value" id="val-filter" class="anchored"><a href="#val-filter" class="anchor"></a><code><span><span class="keyword">val</span> filter : <span><a href="#type-t">t</a> <span class="arrow">&#45;&gt;</span></span> <span>f:<span>( <span><a href="#type-elt">elt</a> <span class="arrow">&#45;&gt;</span></span> bool )</span> <span class="arrow">&#4