, { x, y in
    x + y
})
// numberSum == 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 <code>initialResult</code>---<code>0</code>
in this case---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 sequence is exhausted,