as syntax sugar for a `match` that
runs code when the value matches one pattern and then ignores all other values.
-->
<p>言い換えると、<code>if let</code>は値が一つのパターンにマッチした時にコードを走らせ、
他は無視する<code>match</code>への糖衣構文と考えることができます。</p>
<!--
We can include an `else` with an `if let`. The block of code that goes with the
`else` is the same as the block of code that would go with the `_` case in the
`match` expr