This stack overflow post

… is written by someone confused by the documentation for `filter-map`. I went 
and read the documentation, and *I* was confused for about 30 seconds. I 
eventually proposed rewriting the existing

Returns (filter (lambda (x) x) (map proc lst ...)), but without building the 
intermediate list.


Returns (filter not-false? (map proc lst ...)), but without building the 
intermediate list, where not-false? can be defined as (lambda (x) x).

This text is kludgier, but I think that the use of (lambda (x) x) as 
“not-false?” is idiomatic and confusing. And yes, I realize that this 
suggestion probably applies to many places in the docs. Maybe I should just 
propose adding `not-false?` as a library function, defined as (lambda (x) x)….


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