Fix endl handling inside groupings.

Example:
(b
 [0])

baron/inner_formatting_grouper.py

@@ -153,6 +153,7 @@ def group_generator(sequence):
             in_grouping_mode += 1
         elif current[0] in QUIT_GROUPING_MODE:
             in_grouping_mode -= 1
+            assert in_grouping_mode >= 0
 
         if in_grouping_mode:
             if current[0] in GROUP_THOSE:
@@ -168,11 +169,15 @@ def group_generator(sequence):
                         yield next(iterator)
 
                 fail_on_bad_token(iterator.show_next(), debug_file_content, in_grouping_mode)
+
                 current = append_to_token_before(next(iterator), to_group)
 
+                if current[0] in ENTER_GROUPING_MODE:
+                  in_grouping_mode += 1
                 # TODO test
                 if current[0] in QUIT_GROUPING_MODE:
                     in_grouping_mode -= 1
+                    assert in_grouping_mode >= 0
                     yield current
                     continue
 

tests/test_inner_formatting_grouper.py

@@ -1,5 +1,5 @@
 #!/usr/bin/python
-# -*- coding:Utf-8 -*-
+# -*- coding:utf-8 -*-
 
 from baron.inner_formatting_grouper import group
 
@@ -183,3 +183,27 @@ def test_number_backslash_newline():
         ('INT', '3'),
         ('SPACE', '\\\n'),
     ]
+
+
+def test_nested_grouping_after_endl():
+    """
+    (b
+     [0])
+    """
+    assert group([
+        ('LEFT_PARENTHESIS', '('),
+        ('NAME', 'b'),
+        ('ENDL', '\n'),
+        ('SPACE', ' '),
+        ('LEFT_SQUARE_BRACKET', '['),
+        ('INT', '0'),
+        ('RIGHT_SQUARE_BRACKET', ']'),
+        ('RIGHT_PARENTHESIS', ')'),
+    ]) == [
+        ('LEFT_PARENTHESIS', '('),
+        ('NAME', 'b'),
+        ('LEFT_SQUARE_BRACKET', '[', [('ENDL', '\n'), ('SPACE', ' ')], []),
+        ('INT', '0'),
+        ('RIGHT_SQUARE_BRACKET', ']'),
+        ('RIGHT_PARENTHESIS', ')'),
+    ]