module User_and_group: User_and_grouptype t 
include Identifiable.S
val create : user:string -> group:string -> t
val user : t -> string
val group : t -> string
val for_this_process : unit -> t Or_error.tt for the current process.  If you're using async, there is a wrapper,
    Async.User_and_group, that doesn't do blocking calls.val for_this_process_exn : unit -> t
module Stable:sig..end