 it to the variable
`expensive_closure`. To define a closure, we start with a pair of vertical
pipes (`|`), inside which we specify the parameters to the closure; this syntax
was chosen because of its similarity to closure definitions in Smalltalk and
Ruby. This closure has one parameter named `num`: if we had more than one
parameter, we would separate them with commas, like `|param1, param2|`.
-->
<p>クロージャ定義が<code>=</code>に続き、変数<code>expensive_closure</code>に代入されてい