Flagsmodule type Make_arg = sig ... endmodule type S = sig ... endval create : bit:int -> Core_kernel.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 -.