hanged : 
  <span><span><span class="type-var">'a</span> option</span> <span class="arrow">&#45;&gt;</span></span>
  <span>equal:<span>(<span><span class="type-var">'a</span> <span class="arrow">&#45;&gt;</span></span> <span><span class="type-var">'a</span> <span class="arrow">&#45;&gt;</span></span> bool)</span> <span class="arrow">&#45;&gt;</span></span>
  <span>f:<span>(<span><span class="type-var">'a</span> <span class="arrow">&#45;&gt;</span></span> <span class="type-var">'a</span>)</span> <span class=