Well, using negative indexes for fields can be thought as a new feature with all the consequences mentioned before BUT negative indexes for accessing elements from a sequence, IMHO, is something that anyone would expected to work. That's why at first I thought it was a bug and I fill an issue about it.

The code that parses the fields and the indexes is the same, so when I change it to accept negative indexes, it worked for both cases. I'm attaching a patch that checks if a negative index is used in a field and reverts to the old behavior in that case, allowing only negative indexes for accessing sequences ( "{-1}" will raise KeyError because it will be threated as '-1').

Perhaps in this way this issue could be partially fixed.