WORST_CASE(?,O(n^1))

Solution:
---------

  0 :: [] -(0)-> A(0)
  0 :: [] -(0)-> A(2)
  S :: [A(0)] -(0)-> A(0)
  S :: [A(2)] -(2)-> A(2)
  dbl :: [A(0) x A(0)] -(1)-> A(0)
  unsafe :: [A(2)] -(1)-> A(0)


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

  0 :: [] -(0)-> A_cf(0)
  S :: [A_cf(0)] -(0)-> A_cf(0)
  dbl :: [A_cf(0) x A_cf(0)] -(0)-> A_cf(0)
  unsafe :: [A_cf(0)] -(0)-> A_cf(0)