This is explained in the generators with s/and section in http://clojure.org/guides/spec

You should probably also look at s/int-in for int ranges with gen support

I'm curious if it's an eventual goal to make explain's :in path work with core functions like get-in? Right now the spec'ing of maps as a sequential collection via (every (tuple k? v?)) is causing some ambiguity in the :in path.