(defn group-by
"Returns a map of the elements of coll keyed by the result of
f on each element. The value at each key will be a vector of the
corresponding elements, in the order they appeared in coll."
{:added "1.2"
:static true}
[f coll]
(persistent!
(reduce
(fn [ret x]
(let [k (f x)]
(assoc! ret k (conj (get ret k []) x))))
(transient {}) coll)))