diff --git a/src/data_objects.py b/src/data_objects.py
index abcbde0..ebb62d5 100644
--- a/src/data_objects.py
+++ b/src/data_objects.py
@@ -39,13 +39,17 @@ class NavPoint:
 These are data structures which form mapping to livecarta json structure.
 """
 
-atom = lambda x: not isinstance(x, list)
-nil = lambda x: not x
-car = lambda x: x[0]
-cdr = lambda x: x[1:]
-cons = lambda x, y: x + y
 
-flatten = lambda x: [x] if atom(x) else x if nil(x) else cons(*map(flatten, [car(x), cdr(x)]))
+def flatten(x):
+    """ magic function from stackoverflow for list flattening """
+    atom = lambda i: not isinstance(i, list)
+    nil = lambda i: not i
+    car = lambda i: i[0]
+    cdr = lambda i: i[1:]
+    cons = lambda i, y: i + y
+
+    res = [x] if atom(x) else x if nil(x) else cons(*map(flatten, [car(x), cdr(x)]))
+    return res
 
 
 class ChapterItem: