除了少数的特定异常外，XSLT 通常假定样式表中的空格是无意义的。

XSLT normally assumes that whitespace in a stylesheet isn't meaningful, with a few specific exceptions.

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如果我们已经向 Leaf 类添加了 getValue 方法，我们要么是不得不返回一些类型的标志值表明此方法应用是无意义的，要么是实际抛出一个异常。

If we had added a getValue method to class Leaf, we would either have to return some sort of flag value indicating that the method application is non-sensical or actually throw an exception.

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