scalaz
https://github.com/ekmett/comonad/blob/v4.2.7.2/src/Control/Comonad/Trans/Traced.hs
(Since version 2.8.0) use productIterator instead
https://github.com/ekmett/comonad/blob/v4.2.7.2/src/Control/Comonad/Trans/Traced.hs