To establish default values for arguments which are not specified on the command line, the DO file can begin with the special command $D followed by the defaults for $0, $1, etc. Arguments not specified in this list will have no default. You can also specifically skip over a default argument by inserting just a $ in the argument’s position on the $D line. For example:

$D DEF0 $ DEF2

This would set the default for $0 to "DEF0", the default for $1 to nothing, and the default for $2 to "DEF2".

The $D command, if present, must be on the first line of the DO file.