diff --git a/mapytex/calculus/API/expression.py b/mapytex/calculus/API/expression.py
index 3a921a7..114f290 100644
--- a/mapytex/calculus/API/expression.py
+++ b/mapytex/calculus/API/expression.py
@@ -88,95 +88,6 @@ class Expression(object):
     def __repr__(self):
         return f"<Exp: {renders[self.RENDER](self._tree)}>"
 
-    def balance(self):
-        """ Balance selt._tree in order to optimize end tree number
-
-        # TODO: their will be issue with / which is not commutative |lun. oct.  1 15:03:21 CEST 2018
-
-        :return: optmized version of self
-
-        :example:
-        >>> t = Expression.from_str("1+2+3+4+5+6+7+8+9")
-        >>> print(t._tree)
-        +
-         > +
-         | > +
-         | | > +
-         | | | > +
-         | | | | > +
-         | | | | | > +
-         | | | | | | > +
-         | | | | | | | > 1
-         | | | | | | | > 2
-         | | | | | | > 3
-         | | | | | > 4
-         | | | | > 5
-         | | | > 6
-         | | > 7
-         | > 8
-         > 9
-        >>> t.balance()
-        >>> print(t._tree)
-        +
-         > +
-         | > +
-         | | > 1
-         | | > 2
-         | > +
-         | | > 3
-         | | > 4
-         > +
-         | > +
-         | | > 5
-         | | > 6
-         | > +
-         | | > 7
-         | | > +
-         | | | > 8
-         | | | > 9
-        >>> t = Expression.from_str("1+2+3+4+5*6*7*8*9")
-        >>> print(t._tree)
-        +
-         > +
-         | > +
-         | | > +
-         | | | > 1
-         | | | > 2
-         | | > 3
-         | > 4
-         > *
-         | > *
-         | | > *
-         | | | > *
-         | | | | > 5
-         | | | | > 6
-         | | | > 7
-         | | > 8
-         | > 9
-        >>> t.balance()
-        >>> print(t._tree)
-        +
-         > +
-         | > 1
-         | > 2
-         > +
-         | > 3
-         | > +
-         | | > 4
-         | | > *
-         | | | > *
-         | | | | > 5
-         | | | | > 6
-         | | | > *
-         | | | | > 7
-         | | | | > *
-         | | | | | > 8
-         | | | | | > 9
-
-        """
-        self._tree = AssocialTree.from_any_tree(self._tree).balance()
-
-
     def simplify(self, optimize=True):
         """ Compute as much as possible the expression
 
@@ -195,7 +106,7 @@ class Expression(object):
         """
         if optimize:
             try:
-                self.balance()
+                self._tree = self._tree.balance()
             except AttributeError:
                 pass
         try:
@@ -224,9 +135,9 @@ class Expression(object):
         >>> for s in f.explain():
         ...     print(s)
         1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
-        3 + 7 + 11 + 7 + 17
-        10 + 11 + 24
-        10 + 35
+        3 + 3 + 9 + 6 + 7 + 17
+        6 + 9 + 6 + 24
+        15 + 30
         45
         >>> e = Expression.from_str("1+2+3+4+5+6+7+8+9")
         >>> f_no_balance = e.simplify(optimize=False)
@@ -247,9 +158,8 @@ class Expression(object):
         ...     print(s)
         1 + 2 + 3 + 4 + 5 * 6 * 7 * 8 * 9
         3 + 3 + 4 + 30 * 7 * 72
-        6 + 4 + 30 * 504
-        6 + 4 + 15120
-        6 + 15124
+        6 + 4 + 210 * 72
+        10 + 15120
         15130
         >>> e = Expression.from_str("1+2+3+4+5*6*7*8*9")
         >>> f_no_balance = e.simplify(optimize=False)