diff --git a/4a1s/MFES/PL - Aula 9.md b/4a1s/MFES/PL - Aula 9.md
index 55758aa..7c9ff54 100644
--- a/4a1s/MFES/PL - Aula 9.md	
+++ b/4a1s/MFES/PL - Aula 9.md	
@@ -86,3 +86,101 @@ end
 ```
 
 
+```c
+
+(* 
+    The selection sort algorithm sorts an array 
+    by successively placing in each position the 
+    “next minimum” element, as follows:
+
+    [40, 20, 10, 30, 60, 0, 80]
+    [0, 20, 10, 30, 60, 40, 80]
+    [0, 10, 20, 30, 60, 40, 80]
+    [0, 10, 20, 30, 60, 40, 80]
+    [0, 10, 20, 30, 60, 40, 80]
+    [0, 10, 20, 30, 40, 60, 80]
+    [0, 10, 20, 30, 40, 60, 80]
+    [0, 10, 20, 30, 40, 60, 80]
+
+ 1. Complete the contract of function swap, writing 
+    adequate pre- and post-conditions.
+    Recall that "length a" gives you the allocated length
+    of array a.
+ 2. Write a contract for function select. 
+    Write also one or more loop invariants that allow
+    you to prove the correctness of the function. 
+    Note that a loop variant is not required, since 
+    "for" loops are bounded. 
+ 3. Write one or more loop invariants that allow you to prove
+    the correctness of selection_sort_1. 
+    You may need to modify the contract of select in order 
+    for the loop invariant of selection_sort_1 to be proved. 
+ 4. Finally, write the algorithm selection_sort_2, which does
+    not use the function select. It will have a nested loop; 
+    write invariants for both loops and prove the correctness
+    of the algorithm. 
+*)     
+
+
+module SelectionSort
+
+  use int.Int
+  use ref.Ref
+  use array.Array
+  use array.IntArraySorted
+  use array.ArrayPermut
+  use array.ArrayEq
+
+
+
+  let swap (a: array int) (i: int) (j: int) 
+    requires { 0 <= i< length a && 0 <= j< length a }
+    ensures  { (old a)[i] = a[j] && (old a)[j] = a[i] }
+    ensures  { forall k:int. 0<=k<length a -> k<>i -> k<>j -> (old a)[i] = a[j]}
+    ensures  { permut_all (old a) a }
+  = let v = a[i] in
+    a[i] <- a[j];
+    a[j] <- v
+
+
+
+  (* returns the index of a  minimum of 
+     the segment of a between indexes i and length a -1. 
+   *)
+  let select (a: array int) (i: int) : int
+    requires { 0 <= i < length a}
+    ensures  { i <= result < length a }
+    ensures  { forall k:int. 0<=k<length a -> a[result] <= a[k]}
+  = let ref min = i in
+      for j = i + 1 to length a - 1 do
+        invariant { i <= min < j }
+        invariant { forall k:int. i <= k < j -> a[min] <= a[k] }
+        if a[j] < a[min] then min <- j
+      done;
+    min
+
+
+       
+  let selection_sort_1 (a: array int) 
+    ensures { sorted a }
+    ensures { permut_all (old a) a }
+  = for i = 0 to length a - 1 do
+      invariant { ... }
+      let min = select a i
+      in if min <> i then swap a min i
+    done
+
+
+
+
+
+  let selection_sort_2 (a: array int) =
+    ensures { sorted a }
+    ensures { permut_all (old a) a }
+  ...
+
+
+end      
+
+
+```
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