The natural-number? predicate seems to be
unavailable (though there is a remnant of it in the form of
natural-number/c, which is just a contract alias of
exact-nonnegative-integer?). In any case, it surely means
nonnegative integer.
(One might push the envelope here slightly and add
exactness, but this commit does not go that far.)
The broken link showed up as an undefined tag warning for `(tech "comment")` when installing this package.
This seemed like the most straightforward fix, but it's a bit wordy.
Jesse Alama