diff --git a/python/triqs_dft_tools/block_structure.py b/python/triqs_dft_tools/block_structure.py
index d9e42a66..858a4d14 100644
--- a/python/triqs_dft_tools/block_structure.py
+++ b/python/triqs_dft_tools/block_structure.py
@@ -122,6 +122,7 @@ class BlockStructure(object):
                  transformation=None):
         
         # Ensure backwards-compatibility with pre-3.1.x gf_structs
+        # on second thought: would be better with gf_struct_flatten i guess... #TODO!
         if gf_struct_sumk != None:
             for gf_struct in gf_struct_sumk:
                 for i, block in enumerate(gf_struct):