[letter, default: 0] += 1
}
// letterCount == [&quot;a&quot;: 5, &quot;b&quot;: 2, &quot;r&quot;: 2, &quot;c&quot;: 1, &quot;d&quot;: 1]</code></pre>

<p>When <code>letters.reduce(into:_:)</code> is called, the following steps occur:</p>

<ol><li>The <code>updateAccumulatingResult</code> closure is called with the initial
accumulating value---<code>[:]</code> in this case---and the first character of
<code>letters</code>, modifying the accumulating value by setting <code>1</code> for the key
<code>&quot;a&q