 maps. <code>O(1)</code>.</p></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-choose_exn"><a href="#val-choose_exn" class="anchor"></a><code><span><span class="keyword">val</span> choose_exn : <span><span><span class="type-var">'a</span> <a href="#type-t">t</a></span> <span class="arrow">&#45;&gt;</span></span> <a href="#type-key">key</a> * <span class="type-var">'a</span></span></code></div><div class="spec-doc"><p>Find an unspecified binding. Different bindings may be chosen for