Grabbing Standard Input in Unix Pascal Programs

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I would like to write a few simple pascal programs that receive and
manipulate standard input, but I am not sure how this is done
exactly.  

Is standard input passed to a pascal program as a variable?
Do I grab it with a `readln'?

If anybody knows an answer, or can point me in the right direction,
I would be much obliged.

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Paul Lange
P...@uts.cc.utexas.edu