Errata

Quote:
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Observable<String> squares = files
.flatMap(file -> ranks.map(rank -> file + rank));
The squares Observable will emit exactly 64 events: for 1 it generates a1, a2… a8, followed by b1, b2… to finally reach h7 and h8.
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Since we just learned that flatMap() doesn't preserve the order, wouldn't it be better to use concatMap() instead?

The suggested order a1..a8, b1..b8 etc is technically not guaranteed by flatMap(). I guess it works here because there are no schedulers involved.

concatMap() seems more robust in this case, since it would guarantee the explained order, no matter what schedulers are used.