可以按以下两种形式提供规范：作为完整的查询字符串，或仅作为任务的名称。
The specification can be given in two forms, either as a complete query string, or just the name of the task.
Predicate是计算结果为true或false的常见查询表达式形式。
Predicate is another common form of query expression that evaluates to either true or false.
采用标准消息的形式进行通信，对有关查询或查询响应的信息进行编码。
Communications take the form of a standard message that encodes information about a query, or a response to a query.
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