Lambda the Ultimate

Hello,

I am looking for papers and a particular paper I used to have on covariance and contravariance.

The paper I used to have many years ago probably about 2006 or 2007 when I originally started looking into type theory and did not realize its relevance had many inference rules for covariance and contravariance using plus and minus symbols within the inference rules.

I am now researching this area and am interested in any papers on this topic.

Many thanks in advance,

Aaron Gray