<a href="../FieldLabel/index.html#type-t">FieldLabel.t</a> <span class="arrow">&#45;&gt;</span></span> <span class="type-var">'accum</span>)</span> <span class="arrow">&#45;&gt;</span></span>
  <span><span class="label">fallback</span>:<span>(<span><span class="type-var">'accum</span> <span class="arrow">&#45;&gt;</span></span> <span class="type-var">'accum</span>)</span> <span class="arrow">&#45;&gt;</span></span>
  <span class="type-var">'accum</span></span></code></div><div class="spec-doc"><p>If a varia