きゅーけーRacket だと natural? は exact-nonnegative-integer? の別名として定義されてる。
4.3.2 Generic Numerics https://docs.racket-lang.org/reference/generic-numbers.html?q=natural%3F#%28def._%28%28lib._racket%2Fmath..rkt%29._natural~3f%29%29
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