>>> take 1 (intersperse undefined ('a' : undefined)) "a"
>>> take 2 (intersperse ',' ('a' : undefined)) "a*** Exception: Prelude.undefined
>>> intersperse ',' "abcde" "a,b,c,d,e"
>>> intersperse 1 [3, 4, 5] [3,1,4,1,5]
intersperse 0 (1 :| [2,3]) == 1 :| [0,2,0,3]