gory</a></span></code></div><div class="spec-doc"><p>Return the category to which the attribute belongs.</p></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-is_undef"><a href="#val-is_undef" class="anchor"></a><code><span><span class="keyword">val</span> is_undef : <span><a href="#type-t">t</a> <span class="arrow">&#45;&gt;</span></span> bool</span></code></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-to_string"><a href="#val-to_string" class="anchor">