replace_letter now steal the history of the letter.

pymath/calculus/abstract_polynom.py

@@ -441,15 +441,30 @@ class AbstractPolynom(Explicable):
         >>> C = A.replace_letter(2)
         >>> C
         < Expression [2, 2, *, 1, +]>
+        >>> e = Expression('2+3').simplify()
+        >>> D = A.replace_letter(e)
+        >>> D
+        < Expression [2, 5, *, 1, +]>
+        >>> for i in D.explain():
+        ...    print(i)
+        2 ( 2 + 3 ) + 1
+        2 \times 5 + 1
 
         """
+        exp_to_replace = Expression(letter)
+        exp_to_replace.steal_history(letter)
+
         postfix_exp = [
-                Expression(letter) if i == self._letter
+                exp_to_replace if i == self._letter
                 else i
                 for i in self.postfix_tokens
                 ]
+        ini_step = Expression.develop_steps(postfix_exp)
 
-        return Expression(postfix_exp)
+        ans = Expression(postfix_exp)
+        ans.this_append_before(ini_step)
+
+        return ans
 
     def __eq__(self, other):
         try: