(flatten) (flatten coll)
Takes any nested combination of sequential things (lists, vectors, etc.) and returns their contents as a single, flat foldable collection.
user=> (require '[clojure.core.reducers :as r])
user=> (into [] (r/flatten [1 [2 3 4 5 6 7] 8]))
[1 2 3 4 5 6 7 8]
user=> (into [] (r/flatten [0 2 [[2 3] 8 [[100]] nil [[nil]]] -2]))
[0 2 2 3 8 100 nil nil -2]
user=> (into [] (r/flatten nil))
[]