> find_or_add_lazy : 
  <span><a href="#type-key">key</a> <span class="arrow">&#45;&gt;</span></span>
  <span><span><span class="type-var">'a</span> <a href="#type-t">t</a></span> <span class="arrow">&#45;&gt;</span></span>
  <span><span class="label">f</span>:<span>(<span>unit <span class="arrow">&#45;&gt;</span></span> <span class="type-var">'a</span>)</span> <span class="arrow">&#45;&gt;</span></span>
  <span>[&gt; <span>`Added of <span class="type-var">'a</span></span> <span><span>| `Found</span> of <sp