树的根是文档本身，每个节点对应于一个元素或字符数据。
The root of the tree is the document itself, and each node corresponds to an element or character data.
对于匹配TOPIC对象的主题字符串，以及主题树中该主题字符串之下、没有定义与之匹配的TOPIC对象的各个主题，此TOPIC对象为它们提供默认设置。
The topic object provides default Settings for the topic string that matches it, as well as topics under it in the topic tree that do not have a topic object defined to match them.
清单17展示了一个示例收集器，它将在我们的树中查找字符串。
Listing 17 shows an example collector that looks for strings in our tree.
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