.Container</code>, i.e. generic definitions of container operations in terms of a <code>fold</code> function.</p><div class="odoc-spec"><div class="spec type anchored" id="type-singleton_or_more"><a href="#type-singleton_or_more" class="anchor"></a><code><span><span class="keyword">type</span> <span>'a singleton_or_more</span></span><span> = </span></code><ol><li id="type-singleton_or_more.Empty" class="def variant constructor anchored"><a href="#type-singleton_or_more.Empty" class="anchor"></a><code><span>