This is a solution for ProjectEuler #13 that avoids "cheating" by resorting to BigInteger.

pe13.clj

(defn solution [big-string]
  (->> big-string
    (clojure.string/split-lines)                           ; Break string into individual numbers
    (map reverse)                                          ; Reverse the digits in each
    (map (fn [n] (map #(Integer/parseInt (str %)) n)))     ; Turn all characters into actual ints
    (apply map +)                                          ; Sum each column of digits
    (reduce (fn [[digits carry] n]                         ; Fold over the partial sums,
              [(conj digits (rem (+ n carry) 10))          ;   adding the current sum modulo ten to the list of digits
               (quot (+ n carry) 10)]) [() 0])             ;   and taking the quotient over ten as the carry
    ((fn [[digits carry]] (str carry (apply str digits)))) ; Convert back to a single string, last carry at the front.
    (take 10)))                                            ; Take the first ten characters