(lists/flatten [1 [2 [3]] [] 4])
(lists/flatten [1 null [2]])
(lists/flatten [])
(lists/flatten [[] [[]] []])
(lists/flatten [1 2 3])
(lists/flatten [[a a] [false [true]]])
(lists/flatten
  [{items [1 [2]]}
   [
     hello world
     #custom [3 [4]]]
   null])