allow no Explicable in merge_history and improve doctest

pymath/calculus/explicable.py

@@ -125,6 +125,8 @@ class Explicable(Renderable):
     def merge_history(expl1, expl2, operation):
         r""" Take two Explicable objects, explain where they are from and merge their history
 
+        This method try to use 'explain' method from expl1 and expl2. This means that after using this method, those two objects will become amnesiac.
+
         :param expl1: fist explicable
         :param expl2: second explicable
         :param operation: the operation that link them
@@ -146,11 +148,30 @@ class Explicable(Renderable):
         ['2 + 10 \\times 4', '2 \\times 12', +]
         ['2 + 40', '24', +]
         ['42', '24', +]
+        >>> for i in action1.explain():
+        ...    print(i)
 
+        >>> action1 = Explicable()
+        >>> action1.postfix_tokens = Expression('42')
+        >>> action1.this_append_before([Expression('2+10*4'), Expression('2+40')])
+        >>> m_history = Explicable().merge_history(action1, 12, op.add)
+        >>> m_history
+        <generator object transpose_fill ...>
+        >>> for i in m_history:
+        ...    print(i)
+        ['2 + 10 \\times 4', 12, +]
+        ['2 + 40', 12, +]
+        ['42', 12, +]
         """
-        steps1 = list(expl1.explain())
-        steps2 = list(expl2.explain())
+        try:
+            steps1 = list(expl1.explain())
+        except AttributeError:
+            steps1 = [expl1]
 
+        try:
+            steps2 = list(expl2.explain())
+        except AttributeError:
+            steps2 = [expl2]
         return transpose_fill([steps1, steps2, [operation]])