diff --git a/mapytex/calculus/API/__init__.py b/mapytex/calculus/API/__init__.py
index 2fe24f1..6c50b5b 100644
--- a/mapytex/calculus/API/__init__.py
+++ b/mapytex/calculus/API/__init__.py
@@ -83,16 +83,37 @@ x^7
 
 >>> e = Expression.from_str("2x^2+2x+3x")
 >>> e_simplified = e.simplify()
+>>> print(e_simplified)
+2x^2 + 5x
+>>> for s in e_simplified.explain():
+...    print(s)
+2x^2 + 2x + 3x
+2x^2 + (2 + 3) * x
+2x^2 + 5x
 """
 
 from .expression import Expression
 
 if __name__ == "__main__":
-    e = Expression.from_str("2x^2+3x+4x^2")
-    print(e)
-    e_simplified = e.simplify()
-    print(e_simplified)
-    
+    e = Expression.from_str("2x^2+2x+3x")
+    print(e._tree.map_on_leaf(lambda x:(x,)))
+    for _ in range(5):
+        print("-"*10)
+        e = e._optimize()
+        try:
+            print(e._tree.map_on_leaf(lambda x:(x,)))
+        except AttributeError:
+            print(e)
+        e = e._typing()
+        try:
+            print(e._tree.map_on_leaf(lambda x:(x,)))
+        except AttributeError:
+            print(e)
+        e = e._compute()
+        try:
+            print(e._tree.map_on_leaf(lambda x:(x,)))
+        except AttributeError:
+            print(e)
 
 
 # -----------------------------
diff --git a/mapytex/calculus/API/expression.py b/mapytex/calculus/API/expression.py
index bb16279..01b91fa 100644
--- a/mapytex/calculus/API/expression.py
+++ b/mapytex/calculus/API/expression.py
@@ -91,6 +91,37 @@ class Expression(object):
     def _optimize(self, exclude_nodes=["\\", "**"]):
         """ Return a copy of self with an optimize tree
         
+        :example:
+        >>> e = Expression.from_str("2x^2+2x+3x")
+        >>> print(e._tree)
+        +
+         > +
+         | > *
+         | | > 2
+         | | > ^
+         | | | > x
+         | | | > 2
+         | > *
+         | | > 2
+         | | > x
+         > *
+         | > 3
+         | > x
+        >>> print(e._optimize()._tree)
+        +
+         > *
+         | > 2
+         | > ^
+         | | > x
+         | | > 2
+         > +
+         | > *
+         | | > 2
+         | | > x
+         | > *
+         | | > 3
+         | | > x
+
         """
         try:
             # TODO: need to test exclude_nodes |ven. oct.  5 08:51:02 CEST 2018
@@ -180,7 +211,8 @@ class Expression(object):
         comp_exp = typed_exp._compute()
 
         if typed_exp == comp_exp:
-            return self
+            typed_exp.set_ancestor(self._ancestor)
+            return typed_exp
         else:
             comp_exp.set_ancestor(self)
             return comp_exp.simplify(optimize=optimize)
diff --git a/mapytex/calculus/core/__init__.py b/mapytex/calculus/core/__init__.py
index 1adcf5c..fe95811 100644
--- a/mapytex/calculus/core/__init__.py
+++ b/mapytex/calculus/core/__init__.py
@@ -35,9 +35,11 @@ Abstracts tools for calculs manipulations
  | > 3
  | > 4
 >>> print(t.apply_on_last_level(compute))
-Traceback (most recent call last):
-    ...
-NotImplementedError: Can't divide 2 int. Need to create a Fraction instead
++
+ > 2
+ > /
+ | > 3
+ | > 4
 >>> tt = t.apply_on_last_level(typing)
 >>> print(tt.apply_on_last_level(compute))
 +
diff --git a/mapytex/calculus/core/tree.py b/mapytex/calculus/core/tree.py
index beb65b3..a6157eb 100644
--- a/mapytex/calculus/core/tree.py
+++ b/mapytex/calculus/core/tree.py
@@ -330,6 +330,8 @@ class Tree(object):
         try:
             left_applied = self.left_value.\
                 apply_on_last_level(function)
+        except NotImplementedError:
+            left_applied = self.left_value
         except AttributeError:
             left_applied = self.left_value
             left_is_leaf = 1
@@ -338,6 +340,8 @@ class Tree(object):
         try:
             right_applied = self.right_value.\
                 apply_on_last_level(function)
+        except NotImplementedError:
+            right_applied = self.right_value
         except AttributeError:
             right_applied = self.right_value
             right_is_leaf = 1