一卷意味着该领域内的任何时候，仍有空间，不留白。
A volume means at any point inside that sphere , there is still sphere, not empty space.
就拿一张地图来说，上面的每一点信息都有可能成为考试内容，稍不留神错过一点的话很可能那张图就白读了。
Take a map for every point above the test content information can become too easy to miss the point, then it may be read on the white picture is.
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