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Postscript is a programming language (in reverse polish notation, see RPN in Tcl) that is used mostly for representing printable documents. More decent printers usually understand Postscript right away.
Popular tools to visualize Postscript files are ghostscript/ghostview.
A typical Postscript file starts with a % comment:
%! %%Creator: Gif2PS /#copies 1 def
The last line is as assignment: "assign to the variable #copies the value 1", or set #copies 1 as we would say in Tcl.
See also Manipulating Postscript