diff --git a/4a1s/MFES/PL - Aula 9.md b/4a1s/MFES/PL - Aula 9.md
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--- a/4a1s/MFES/PL - Aula 9.md	
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+```c
+
+(* 
+    Write appropriate definitions and specifications for
+    functions split and merge, used by the merge sort 
+    algorithm defined below in the module. 
+    
+    Note that the verification of mergesort relies only 
+    on the specifications ("contracts") of the auxiliary 
+    functions, and not on their code. As such, it is crucial
+    to include in them every aspect required for the
+    verification conditions of the sorting algorithm
+    to be successfully discharged.
+    
+    Note also that although Why3 is capable of automatically 
+    proving the termination of functions with simple recursive
+    patterns, that does not apply to the mergesort function. 
+    We thus include in the contract the variant "length l", 
+    which is valid since the function is recursively called 
+    on lists that are shorter than l. 
+*)    
+
+
+
+module MergeSort
+
+  use int.Int
+  use list.List
+  use list.Length
+  use list.Permut
+  use list.Append
+  use list.SortedInt
+
+
+
+  let rec function split (l :list int) : (list int, list int)
+    ensures { let (l1, l2) = result in permut l (l1++l2) }
+    ensures {let (l1,l2) = result in length l<2 || (length l>=2 && 
+        length l1<length l2 && length l2 < length l1)}
+  = match  l with 
+        | Nil -> (Nil, Nil)
+        | Cons x Nil -> (l, Nil)
+        | Cons x (Cons y t) -> let (l1, l2) = split t
+            in (Cons x l1, Cons y l2) 
+  end
+
+
+
+  let rec function merge (l1 l2 :list int) : list int
+    requires { sorted l1 && sorted l2 }
+    ensures  { sorted result} 
+    ensures {permut result (l1++l2)}
+  = match l1, l2 with
+    Nil, _ -> l2
+    | _, Nil -> l1
+    | (Cons x l1'), (Cons y l2') -> if x<=y
+        then Cons x (merge l1' l2)
+        else Cons y (merge l1 l2')
+  end
+
+
+  
+  let rec function mergesort (l :list int)
+    ensures { sorted result } 
+    ensures { permut l result } 
+    variant { length l }
+  =
+    match l with
+    | Nil -> Nil
+    | Cons x Nil -> Cons x Nil
+    | _ ->   let (l1,l2) = split l
+             in merge (mergesort l1) (mergesort l2)
+    end
+
+
+
+
+  predicate is_a_sorting_algorithm (f: list int -> list int) = 
+    forall al :list int. permut al (f al) /\ sorted (f al)
+
+  goal is_a_sorting_algorithm : is_a_sorting_algorithm mergesort
+
+
+
+end
+```
+
+