WORST_CASE(?,O(n^1))

Solution:
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  "0" :: [] -(0)-> "Nat"(1)
  "minus" :: ["Nat"(0) x "Nat"(1)] -(1)-> "Nat"(0)
  "s" :: ["Nat"(0)] -(0)-> "Nat"(0)
  "s" :: ["Nat"(1)] -(1)-> "Nat"(1)


Cost Free Signatures:
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