module type Make_arg = Flags_intf.Make_argval known : (Flags_intf.Int63.t * string) listflag, name in known means that the flag bit is called name.
known is only used to make sexp_of_t's output human readable.
The flags in the output of sexp_of_t will occur in the same order as they appear
in known.
It is allowed to have a single flag with multiple bits set.
It is an error if there are different flags with the same bit set.