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From: Helge Blischke <h.blischke@acm.org>
Newsgroups: comp.lang.postscript
Subject: Re: What is the best way to work with a dictionary
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Date: Sat, 13 Jul 2013 22:32:50 +0200
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Cecil Westerhof wrote:

> Until now I wrote functions like:
>     /ellipse { % x y unitX unitY colour
>         5 dict begin
>             /colour  exch def
>             /unitY   exch def
>             /unitX   exch def
>             /y       exch def
>             /x       exch def
>     
>             gsave
>             newpath
>             colour  setrgbcolor
>             x  y  translate
>             unitX  unitY  scale
>             1  1  1  0  360  arc closepath fill
>             stroke
>             grestore
>         end
>     } def
> 
> I just saw in ‘PostScript Drawing: An Economist's Guide’ that it also
> should be possible to do it in the following way:
>     /ellipse { % x y unitX unitY colour
>         <<
>             /colour  exch 
% operand stack at this point:
% x y unitX unitY color /color mark
>		             def
>             /unitY   exch def
>             /unitX   exch def
>             /y       exch def
>             /x       exch def
>         >>  begin
>             gsave
>             newpath
>             colour  setrgbcolor
>             x  y  translate
>             unitX  unitY  scale
>             1  1  1  0  360  arc closepath fill
>             stroke
>             grestore
>         end
>     } def
> 
> Which of the two is the better one and why? I think I like the second
> one more.
> The only problem is that it does not work. I get:
>     Error: /unmatchedmark in -->>--
> 
> I am using:
>     GPL Ghostscript 9.06
> 

You see that the name /color gets associated with the object mark, i. e. the
--<<--, which is equivalent to --mark--, gets consumed and therefore is 
missing at the time -->>-- is executed.

Think about it.

Helge