HLL65F - INDEN/UNDEN

Notes: Since writing this post I have solved the mystery of the two cryptic lines, I wrote another post about this: 

https://dansdigitalarchaeology.blogspot.com/2022/09/hll65f-indenunden-mystery-solved.html

In my last post I introduced the HLL65 high level language 6502 macro library used in the Atari Crystal Castles source code. The macro definitions used macros called INDEN and UNDEN that I assume are for handling indentation of the code in the listing file. Exactly how these work is a little but of a mystery to me. Here is the code for INDEN:

.MACRO $INDEN
.PUSH REGSAV,...S1

.IF LT,..NST$
.ERROR ..NST$ ; STACK UNDERFLOW
..NST$ = 0
.ENDC

...S1 = ..NST$+1*3+..SRC$
.IIF GT,...S1-<9.*3+..SRC$>,...S1 = 9.*3+..SRC$
.LIST SRC(...S1,1)
.PRINT ..NST$(37,1,16,1,38'->')
..NST$ = ..NST$ + 1
.POP REGSAV,...S1
.ENDM

Like most macros is starts by saving a variable on the stack. The ..NST$ variable is used to keep track of the depth of the indent, so the next block of lines checks to be sure the code hasn't undented further then it has indented and if it did it throws an assembler error. 

When the macro library is initialized ..SRC$ is set to 41, so the next line sets ...S1 to 41 + 3 times the current number of indents. The next line checks if ...S1 is beyond 9 indents and if it is, it sets it back to the value for 9 indents. 

The next line is the first one I don't understand. According to documentation I found for a VAX assembler, .LIST is used to control what gets displayed in the listing, but the documentation does not list the argument syntax used in this code. It's possible this is displaying the current line at a specific place on the line. 

The .PRINT pseudo-op is used to write text to the listing output, but the rest of this line is quite cryptic. The string '->' makes sense as an indication of indentation, but I am not sure what the numbers before it do. I also don't understand the parenthesis right after the ..NST$ variable. 

The next line increments the indent counter and finally the value of ...S1 is restored. 

The OUTDEN macro works the exact same way as this macro, but it just reduces the indent. 

If anyone has any ideas about the two cryptic lines, let me know.