module type Flags =sig..end
module type Make_arg = Flags_intf.Make_argmodule type S = Flags_intf.Sval create : bit:int -> Flags_intf.Int63.tcreate ~bit:n creates a flag with the nth bit set.  n must be between 0 and
      62.
      Typically a flag has one bit set; create is useful in exactly those cases.  For
      flags with multiple bits one can either define the Int63.t directly or create it in
      terms of simpler flags, using + and -.
module Make:
Flags.Make builds a new flags module.