diff --git a/pymath/calculus/expression.py b/pymath/calculus/expression.py
index 99855fb..79e8ace 100644
--- a/pymath/calculus/expression.py
+++ b/pymath/calculus/expression.py
@@ -243,14 +243,15 @@ class Expression(Explicable):
             tmpTokenList += tokenList
             self.child = Expression(tmpTokenList)
 
-            steps = self.develop_steps(tmpTokenList)
+            steps = Expression.develop_steps(tmpTokenList)
 
             if self.child.postfix_tokens == ini_step.postfix_tokens:
                 self.child.steps = steps
             else:
                 self.child.this_append_before([ini_step] + steps)
 
-    def develop_steps(self, tokenList):
+    @classmethod
+    def develop_steps(cls, tokenList):
         """ From a list of tokens, it develops steps of each tokens """
         tmp_steps = []
         for t in tokenList:
@@ -264,18 +265,15 @@ class Expression(Explicable):
             except AttributeError:
                 tmp_steps.append([t])
 
-        if max([len(i) for i in tmp_steps]) == 1:
-            # Cas où rien n'a dû être expliqué.
-            return []
-        elif min([len(i) for i in tmp_steps]) == 0:
-            raise ValueError('tmp_steps: {} \ntokenList: {}'.format(tmp_steps, tokenList))
-        else:
+        if max([len(i) for i in tmp_steps]) > 1:
             tmp_steps = expand_list(tmp_steps)[:-1]
             steps = [Expression(s) for s in tmp_steps]
             return steps
 
+        return []
+
     @classmethod
-    def isExpression(self, other):
+    def isExpression(cls, other):
         try:
             other._isExpression
         except AttributeError: