(defn index-filter [pred coll]
  (when pred 
    (for [[idx elt] (indexed coll) :when (pred elt)] idx)))

(index-filter #{\a \b} "abcdef") 
-> (0 1)

but I don't know why use "when pred" in the code, 
and why "(index-filter {\a \b} "abcdef")" doesn't work?

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