If you are parsing/manipulating a particularly hairy data structure, try to simplify it. If you cannot, put a comment with a simplified example of the input/out

If you are parsing/manipulating a particularly hairy data structure, try to simplify it. If you cannot, put a comment with a simplified example of the input/output data structure(s) so that the the next developer (which may be you few months down the line) has something visual to match the code against instead of having to imagine everything.