Thank you for the explanation of polarity, I found it helpful. I just remembered that people use the +/- notation to denote covariance and contravariance (such

Thank you for the explanation of polarity, I found it helpful. I just remembered that people use the +/- notation to denote covariance and contravariance (such as in OCaml syntax and Scala syntax). I think it's possible that the author saw this and then related the +/- notation to polarity, even though variance is unrelated.