remove steal_history in init for Expression

pymath/calculus/explicable.py

@@ -131,6 +131,22 @@ class Explicable(Renderable):
 
         :param arg1: a potential Explicable
 
+        >>> e = Explicable(['Actions'])
+        >>> s = [Step(['eat']), Step(['sleep'])]
+        >>> e.this_append_before(s)
+        >>> f = Explicable(['Stolen actions'])
+        >>> f.steal_history(e)
+        >>> for i in e.explain():
+        ...    print(i)
+        Actions
+        >>> for i in f.explain():
+        ...    print(i)
+        eat
+        sleep
+        Actions
+        Stolen actions
+
+
         """
         try:
             with Step.tmp_render():
@@ -195,7 +211,6 @@ class Explicable(Renderable):
         ...    print(i)
         [< Step [1]>, < Step [2]>, < Step [3]>]
         """
-        from .expression import Expression
         steps = []
         for expl in explicables:
             try:

pymath/calculus/expression.py

@@ -70,7 +70,6 @@ class Expression(Explicable):
                 )
 
         super(Expression, self).__init__(pstf_tokens)
-        self.steal_history(exp)
         self._isExpression = 1
 
     def simplify(self):