Batteries user guide : BatSet.S.Infix

`sig val ( <-- ) : BatSet.S.t -> BatSet.S.elt -> BatSet.S.t val ( <. ) : BatSet.S.t -> BatSet.S.t -> bool val ( >. ) : BatSet.S.t -> BatSet.S.t -> bool val ( <=. ) : BatSet.S.t -> BatSet.S.t -> bool val ( >=. ) : BatSet.S.t -> BatSet.S.t -> bool val ( -. ) : BatSet.S.t -> BatSet.S.t -> BatSet.S.t val ( &&. ) : BatSet.S.t -> BatSet.S.t -> BatSet.S.t val ( ||. ) : BatSet.S.t -> BatSet.S.t -> BatSet.S.t end`