Example of reducers/fold hanging when the input is a hashmap

gistfile1.clj

(let [sqs {"AUGUGGAAAG" 4, "CGACCUCGUC" 2, "CCAGCGAACC" 2, "GGGCGAAGCU" 5, "GUUUACUUCA" 1, 
           "UACCAUAGUA" 1, "CGACCAGCAU" 2, "UGAAUUUCGA" 2, "GGUCUUUCUU" 2, "UGAAAUACAU" 1}]
                                 (prn :>>)
                                 (r/fold (fn ([] {})
                                           ([l r] (merge-with + l r)))
                                         (fn [M sq cs]
                                           (->> (take 2 sq)
                                                frequencies
                                                (merge-with + M)))
                                         sqs))