The filter function is non-intuitive

Not really. A filter usually separates something into two parts. It is up to the user to decide what is kept (could be both, just needed to be separate). Eg an aggregate grading sieve would keep all parts, all of which are needed for the analysis.

The lesson is that one should not rely on intuition for these things — just read the docs if you are unsure.

Probably not, see

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