diff --git a/pymath/calculus/expression.py b/pymath/calculus/expression.py
index 4879f1b..840f3af 100644
--- a/pymath/calculus/expression.py
+++ b/pymath/calculus/expression.py
@@ -107,16 +107,15 @@ class Expression(Explicable):
         """
         return super(Expression, cls).tmp_render(render)
 
-    def __new__(cls, exp):
+    def __init__(self, exp):
         """Create Expression objects
 
         :param exp: the expression. It can be a string or a list of postfix tokens.
 
         """
-        expression = object.__new__(cls)
 
         if isinstance(exp, str):
-            expression.postfix_tokens = str2tokens(exp)
+            pstf_tokens = str2tokens(exp)
 
         elif isinstance(exp, list):
             # Ici on ne peut convertir les "+-*/..." en opérateur que s'ils sont
@@ -124,15 +123,18 @@ class Expression(Explicable):
             exp_mod_op = [
                 op.get_op(i) if op.can_be_operator(i) else i for i in exp
                 ]
-            expression.postfix_tokens = flatten_list(
-                [tok.postfix_tokens if Expression.isExpression(tok) else tok for tok in exp_mod_op]
+            pstf_tokens = flatten_list(
+                [tok.postfix_tokens if Expression.isExpression(tok) 
+                    else tok 
+                    for tok in exp_mod_op
+                    ]
                 )
 
         elif isinstance(exp, Expression):
-            return exp
+            pstf_tokens = exp.postfix_tokens
 
         elif isNumerand(exp):
-            expression.postfix_tokens = [exp]
+            pstf_tokens = [exp]
 
         else:
             raise ValueError(
@@ -141,8 +143,9 @@ class Expression(Explicable):
                     )
                 )
 
-        expression._isExpression = 1
-        return expression
+        super(Expression, self).__init__(pstf_tokens)
+        self.steal_history(exp)
+        self._isExpression = 1
 
     def __str__(self):
         """
@@ -269,7 +272,7 @@ class Expression(Explicable):
         [< Expression [1, 2, +, 3, 4, *, 5, +, +, 6, 7, +, *]>, < Expression [3, 12, 5, +, +, 13, *]>]
         """
         with Expression.tmp_render():
-            tmp_steps = list(Explicable().merge_history(tokenList))
+            tmp_steps = list(Explicable.merge_history(tokenList))
 
         tmp_steps = list(tmp_steps)[:-1]
         steps = [Expression(s) for s in tmp_steps]