WORST_CASE(?,O(n^1))

Solution:
---------

  @ :: [a(8) x a(0)] -(8)-> a(0)
  dd :: [a(8) x a(8)] -(8)-> a(8)
  dd :: [a(15) x a(15)] -(15)-> a(15)
  dd :: [a(0) x a(0)] -(0)-> a(0)
  flatten :: [a(15)] -(8)-> a(0)
  nil :: [] -(0)-> a(8)
  nil :: [] -(0)-> a(15)
  nil :: [] -(0)-> a(0)


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)