module type S =sig..end
module type S is the interface for a set of flags.  Values of type t are set of
    flags, and the various functions operate on sets of flags.  There is a finite universe
    of flags (in particular 63 flags, one for each bit).
    sexp_of_t uses the flag names supplied to Flags.Make
type t 
val of_int : int -> tval to_int_exn : t -> intval equal : t -> t -> boolval empty : tval (+) : t -> t -> tval (-) : t -> t -> tval intersect : t -> t -> tval complement : t -> tval do_intersect : t -> t -> boolval are_disjoint : t -> t -> boolval sexp_of_t : t -> Sexplib.Sexp.t