iv class="odoc-spec"><div class="spec value" id="val-union" class="anchored"><a href="#val-union" class="anchor"></a><code><span><span class="keyword">val</span> union : <span><span>( <span><a href="#type-key">key</a> <span class="arrow">&#45;&gt;</span></span> <span><span class="type-var">'a</span> <span class="arrow">&#45;&gt;</span></span> <span><span class="type-var">'a</span> <span class="arrow">&#45;&gt;</span></span> <span><span class="type-var">'a</span> option</span> )</span> <span class="arrow">&#