the Ruby team decided that flip-flop usage would (temporarily) raise a warning in Ruby v2.6 (shown below) and perhaps even be deprecated eventually.
弃用了,不要学
However, it had to come back in version 2.7 onwards upon popular demand. Therefore, to run flip-flop code and follow along with our examples, you will need a Ruby installation with version > v2.7 (or < v2.6).