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From: Helge Blischke <h.blischke@acm.org>
Newsgroups: comp.lang.postscript
Subject: Re: Unbalanced [
Followup-To: comp.lang.postscript
Date: Thu, 12 Sep 2013 11:31:13 +0200
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luser- -droog wrote:

> On Wednesday, September 11, 2013 11:16:13 AM UTC-5, ana...@gmail.com
> wrote:
>> Hi everyone,
>> 
>> I am writing a PostScript interpreter for a product I sell, and while
>> working with some sample .EPS files, I ran into the following:
>> 
>> /xpdfm
>> {
>> {
>> dup 0 get/Label eq
>> {
>> aload length[exch 1 add 1 roll/PAGELABEL
>> }{
>> aload pop
>> [{ThisPage}<<5 -2 roll>>/PUT
>> }ifelse
>> pdfmark_5
>> }forall
>> 
>> 
>> This code snippet contains two unbalanced opening square brackets ([).
>> 
>> Is this normal? How should this be treated by a PostScript interpreter?
>> 
>> Thanks in advance.
>> 
> 
> As others have said, the left bracket [ is defined in systemdict as a
> marktype object (same as the name /mark). Since only one of the procedure-
> bodies passed to ifelse will be executed, there is really only one mark
> without a matching right bracket ]. This style is used in much the same
> way as a vararg function in C, to pass an open-ended set of values,
> presumably to pdfmark_5. I'm not familiar with the '_5' part, but pdfmark
> is used to pass data into a generated pdf file.
> 
> If your output device is not a pdf-writer, you can probably safely
> define
> 
>     /pdfmark { cleartomark } def
> 
> By the way, the << is also a mark, so you will sometimes see [ used
> instead.
> 
>     [ /this (is) /a (dictionary) >>

The ample snippet is from the prolog section Acrobat 
(Adobe Reader as well) generates when printing to PostScript printers, 
and /pdfmark_5 is there defined like:

/currentdistillerparams where{pop currentdistillerparams/CoreDistVersion get 5000 lt}{true}ifelse
{
        /pdfmark_5{cleartomark}bind def
}{
        /pdfmark_5{pdfmark}bind def
}ifelse

Helge