Comment by tsm

No 'm' either:

    (def phrase "Fjord zoologists quip jovially, waxing lyrical about xanthic lutrines")
    (require '[clojure.string :as str])
    (-> phrase str/lower-case distinct sort)
    ;; => (\space \, \a \b \c \d \e \f \g \h \i \j \l \n \o \p \q \r \s \t \u \v \w \x \y \z)

(edit: OP originally just mentioned 'k' but then ninja-edited in the 'm')