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From: Helge Blischke <h.blischke@acm.org>
Newsgroups: comp.lang.postscript
Subject: Re: Unbalanced [
Followup-To: comp.lang.postscript
Date: Wed, 11 Sep 2013 18:34:07 +0200
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anavi8r@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.
> 
> -- Peter

The "[" is a synonym for the "mark" operator. Its purpose is
to process the following objects on the operand stack back
up to the mark (and removing the mark object) without the necessity
to know how many objects to process.
See e.g. the cleartomark operator from the PLRM as a simple
example, or the array operator.

Helge