and the result will be put into `var`.
See 'Option Callbacks' below for a more elaborate description.
+`OPT_FILENAME(short, long, &var, description)`::
+ Introduce an option with a filename argument.
+ The filename will be prefixed by passing the filename along with
+ the prefix argument of `parse_options()` to `prefix_filename()`.
+
`OPT_ARGUMENT(long, description)`::
Introduce a long-option argument that will be kept in `argv[]`.