https://github.com/slamdata/purescript-css/blob/v3.4.0/src/CSS/Selector.purs#L85-L85

Looks like a copy/paste mistake. For reference, in Clay (#) is the infix version of with with the same signature:
http://hackage.haskell.org/package/clay-0.12.2/docs/src/Clay-Selector.html#with