diff --git a/expression.py b/expression.py
index 999a7f5..63412fc 100644
--- a/expression.py
+++ b/expression.py
@@ -43,11 +43,18 @@ polonaise inversée et renvoyant l’arbre (de type Expression) correspondant"""
     pile = p()
     for elem in liste_npi :
         if elem == "+" :
-            exp = Expression("+", Expression(Expression()))                
+            droit = pile.depiler()
+            gauche = p.depiler()
+            exp = Expression(elem, gauche, droit)
+            p.empiler(exp)
         elif elem == "*" :
-            exp = Expression("*", Expression(Expression()))
+            droit = pile.depiler()
+            gauche = pile.depiler()
+            exp = Expression(elem, gauche, droit)
+            pile.empiler(exp)
         else :
-            pass
-    return pile[-1]
+            exp = Expression(elem, None, None)
+            pile.empiler(exp)
+    return p.sommet()
             
             
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