最后六行的上下文不很明确，第3行只表示上下文。
The last six lines are contextually weak, and line 3 represents nothing but context.
某标识表示上下文无关的标识符（比如限定标识）。
X-id is an identifier with no context-dependent meaning (e. g. qualified-id).
subsequence的第二个参数是开始位置，点号表示上下文项。
The second argument to subsequence is the starting position; the dot represents the context item.
应用推荐