>'b</span> <a href="#type-t">t</a></span> <span class="arrow">&#45;&gt;</span></span>
  <span><span class="label">f</span>:<span>(<span><a href="#type-key">key</a> <span class="arrow">&#45;&gt;</span></span> <span><span>[ <span>`Left of <span class="type-var">'a</span></span> <span><span>| `Both</span> of <span class="type-var">'a</span> * <span class="type-var">'b</span></span> <span><span>| `Right</span> of <span class="type-var">'b</span></span> ]</span> <span class="arrow">&#45;&gt;</span></span> <span>