this_append_before for Expression

pymath/calculus/expression.py

@@ -263,7 +263,7 @@ class Expression(Explicable):
             if self.child.postfix_tokens == ini_step.postfix_tokens:
                 self.child.steps = steps
             else:
-                self.child.steps = [ini_step] + steps
+                self.child.this_append_before([ini_step] + steps)
 
     def develop_steps(self, tokenList):
         """ From a list of tokens, it develops steps of each tokens """