Three functions are available for manipulating variable argument lists. The first is num_args(...), which returns the number of arguments with which it was invoked, first expanding any ... ellipsis arguments. The second is nth_arg(n, ...), which returns its n'th argument, numbering n itself as 0. The third is missing( ), which returns a vector with T in each element that corresponds to a missing parameter and F elsewhere.
See § 6.4.4 and § 6.5, for a full discussion of these functions.