Fix safe_father_of_first

seminar/treelib.py

@@ -20,7 +20,11 @@ def safe_pred(node):
 
 # A to samé pro .father_of_first
 def safe_father_of_first(node):
-		return node.prev
+	first_brother = node
+	while safe_pred(first_brother) is not None:
+		first_brother = safe_pred(first_brother)
+	try:
+		return first_brother.father_of_first
 	except ObjectDoesNotExist:
 		return None