A set is called a G delta set if it can be written as the countableintersection of open sets. Any closed set is a G delta set. The complement of a G delta set is called a F sigma set[?]. It is easy to see that any open set is a F sigma set.
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... groups also helped Allied pilots who had been shot down to get back to Britain. They minimized the threat of discovery by adopting a cell structure[?].
Groups ...