Can anyone explain the etymology of それはそうと【其れはそうと】and possibly the grammar of how it fits into a sentence, which does not seem to follow the normal rules (It should have a copula だ、そうだと？)? I think of it as equivalent to ところで. The Apple dictionary defines it as
〔ついでながら〕by the way; incidentally; 〔さて〕well
and for reference, it also gives the example sentences:
それはそうと昨日は中野さんに会いましたか｜ By the way, did you see Miss Nakano yesterday?
それはそうとあの本はやはり山田さんのでした| Incidentally, that book was Mr. Yamada's, just as we thought.
それはそうと次の会合はいつにしますか｜Well, then, when shall we have our next meeting?