在开发模式时，我们为将来的模式元素创建占位符。
As the pattern develops, we create placeholders for future pattern elements.
其中最重要一个就是变量，你可以把它简单的看做是一个占位符，它其中需要存放的数据值在你写代码时是未知的。
Of these, the most important is that of a variable, which is easiest to think of as a placeholder for some piece of data whose value is unknown when the program is written.
当前字体中找不到某个适当的字符时，多数系统会显示一个某种占位符。
Most systems fall back to displaying a placeholder of some sort when an appropriate character can't be found in the current font.
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