module Check_result:sig..end
thread_safe_check t pre ~timeout checks the file descriptors for their status and
      returns when at least one is available, or the timeout passes.
      thread_safe_check does not side effect t.  Unlike the rest of the functions in
      this module, thread_safe_check is thread safe.type t 
val sexp_of_t : t -> Sexplib.Sexp.t