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The following code:
(reduce + (for [x xs] 1)))
triggered the 'count' trip alarm on problem number 22.
The problem is that for is a macro and expands to code that uses count. 4clojure's sandbox has to expand macros to look for blacklisted things. It is an unfortunate side effect that for doesn't work here.
Luckily, you can write it in many different ways.
(fn [xs] (reduce + (map (fn [_] 1) xs)))
Is your exact code written with map. There are other ways to solve this problem, but that's the exercise to the reader. :)
Thanks Raynes, that makes sense - I haven't internalized Clojure's macros' contents yet. My second choice was
(reduce + (map #(1) xs))
which was a bit Pythonesque and didn't work here (no binding for the argument to the lambda, I assume), so it's nice to be pointed out that an anon func with a throw-away would also work.
#(1) doesn't work because it is equivalent to (fn  (1)).
(fn  (1))