File names have a numbered core that communicates the current file number
as well as the total number of files in the set. For example, 02-20 would
indicate the second file in a set of 20. Other customizations can be added.
Usage
file_set(n, where = NULL, prefix = NULL, pad = TRUE, sep = "-", ext = "")
Value
By default a list length n of lists length 2; each sublist contains the
integer file number as i and the file name as file.