The arg-id field specifies the index of
the argument in args
whose value is to be formatted and inserted into the output
instead of the replacement field.
If there is no argument with
the index arg-id in args,
the string is not a format string for args.
The optional format-specifier field
explicitly specifies a format for the replacement value.
If all arg-ids in a format string are omitted
(including those in the format-spec,
as interpreted by the corresponding formatter specialization),
argument indices 0, 1, 2, … will automatically be used in that order.
If some arg-ids are omitted and some are present,
the string is not a format string.