The SET.ARG statement updates a subroutine argument value based on its position in the argument list. It is intended for use with subroutines declared with the VAR.ARGS option.
SET.ARG n, value
Subroutines declared with the VAR.ARGS option may have a variable number of arguments. Although each argument must have a name assigned to it in the SUBROUTINE statement, it is often useful to be able to process a series of arguments by indexing this list.
The SET.ARG statement sets the value of argument n. The actual number of arguments passed may be determined using the ARG.COUNT() function. Use of an argument position value less than one or greater than the number of arguments causes the program to abort.