diff --git a/mapytex/calculus/core/tree.py b/mapytex/calculus/core/tree.py
index 0961beb..29f0da8 100644
--- a/mapytex/calculus/core/tree.py
+++ b/mapytex/calculus/core/tree.py
@@ -296,11 +296,11 @@ class Tree(object):
 
     def apply_on_last_level(self, 
                             function, 
-                            type_function = lambda n,l,r: Tree(n,l,r)):
+                            fallback = lambda n,l,r: Tree(n,l,r)):
         """ Apply the function on last level of the tree before leaf
 
         :param function: (op, a, a) -> b function to apply on last level
-        :param type_function: (op, a, a) -> b  typing function to apply on subtree which raise a TypeError. The return value will be concidered as an leaf
+        :param fallback: (op, a, a) -> b  fallback function to apply on subtree which raise a NotImplementedError. The return value will be concidered as an leaf
         :returns: b if it is a 1 level Tree, Tree otherwise
 
 
@@ -333,12 +333,12 @@ class Tree(object):
         right_is_leaf = 0
         try:
             left_applied = self.left_value.\
-                apply_on_last_level(function, type_function)
+                apply_on_last_level(function, fallback)
         except NotImplementedError:
             # TODO: overload __getitem__ to ease unpacking |dim. sept. 23 19:17:50 CEST 2018
             left_is_leaf = 1
             left_tree = self.left_value
-            left_applied = type_function(left_tree.node,
+            left_applied = fallback(left_tree.node,
                                          left_tree.left_value,
                                          left_tree.right_value)
         except AttributeError:
@@ -347,11 +347,11 @@ class Tree(object):
 
         try:
             right_applied = self.right_value.\
-                apply_on_last_level(function, type_function)
+                apply_on_last_level(function, fallback)
         except NotImplementedError:
             right_is_leaf = 1
             right_tree = self.right_value
-            right_applied = type_function(right_tree.node,
+            right_applied = fallback(right_tree.node,
                                          right_tree.left_value,
                                          right_tree.right_value)
         except AttributeError: