span class="type-var">'acc</span> <span class="arrow">&#45;&gt;</span></span>
  <span><span class="label">f</span>:<span>(<span><span class="type-var">'acc</span> <span class="arrow">&#45;&gt;</span></span> <span><a href="#type-key">key</a> <span class="arrow">&#45;&gt;</span></span> <span><a href="#type-value">value</a> <span class="arrow">&#45;&gt;</span></span> <span class="type-var">'acc</span> * <a href="#type-value">value</a>)</span> <span class="arrow">&#45;&gt;</span></span>
  <span class="type-var"