WORST_CASE(?,O(n^1))

Solution:
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  "@" :: ["a"(12) x "a"(2)] -(14)-> "a"(2)
  "dd" :: ["a"(12) x "a"(12)] -(12)-> "a"(12)
  "dd" :: ["a"(15) x "a"(15)] -(15)-> "a"(15)
  "dd" :: ["a"(2) x "a"(2)] -(2)-> "a"(2)
  "flatten" :: ["a"(15)] -(8)-> "a"(2)
  "nil" :: [] -(0)-> "a"(12)
  "nil" :: [] -(0)-> "a"(15)
  "nil" :: [] -(0)-> "a"(6)


Cost Free Signatures:
---------------------

  "@" :: ["a"_cf(0) x "a"_cf(0)] -(0)-> "a"_cf(0)
  "dd" :: ["a"_cf(0) x "a"_cf(0)] -(0)-> "a"_cf(0)
  "flatten" :: ["a"_cf(0)] -(0)-> "a"_cf(0)
  "nil" :: [] -(0)-> "a"_cf(0)


Base Constructors:
------------------
  "\"dd\"_a" :: ["a"(1) x "a"(1)] -(1)-> "a"(1)
  "\"nil\"_a" :: [] -(0)-> "a"(1)