<span class="type-var">'a</span> option</span>)</span> <span class="arrow">&#45;&gt;</span></span>
  <span><span class="type-var">'a</span> <a href="#type-t">t</a></span></span></code></div><div class="spec-doc"><p>Like merge, but specialized to require <code>f</code> to preserve the type of the left argument, which enables preserving <code>==</code> if <code>f</code> preserves <code>==</code> of every value.</p></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-union"><a href="#val