tro-declaration"><code class="language-swift">func -=(lhs:rhs:)(lhs: inout UInt64, rhs: UInt64)</code></div><div class="discussion comment"><p>Subtracts the second value from the first and stores the difference in the
left-hand-side variable.
</p>
<p>The difference of the two arguments must be representable in the
arguments' type. In the following example, the result of <code>21 - 50</code> is
less than zero, the minimum representable <code>UInt8</code> value:</p>
<pre><code class="language-swift">var x: UI