comment for deleted levels in MI

Author: jreback

pandas/tests/frame/test_mutate_columns.py

@@ -184,6 +184,8 @@ def test_delitem_multiindex(self):
         with pytest.raises(KeyError):
             del df[('A',)]
 
+        # xref: https://github.com/pandas-dev/pandas/issues/2770
+        # the 'A' is STILL in the columns!
         assert 'A' in df.columns
         with pytest.raises(KeyError):
             del df['A']