t) -&gt; Int = { x, y in x + y }
let numberSum = numbers.reduce(0, addTwo)
// &#39;numberSum&#39; == 10</code></pre>

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

<ol><li>The <code>nextPartialResult</code> closure is called with the initial
result and the first element of <code>numbers</code>, returning the sum:
<code>1</code>.</li><li>The closure is called again repeatedly with the previous call&#39;s
return value and each element of the sequence.</li><li>When the se