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Although I do appreciate that you try to improve python it is not clear
to me from http://docs.python.org/lib/module-subprocess.html:
1. Why the old functions have been deprecated
2. I can pipe together two processes. But how can I use pipe to feed the
first process of the two? The example at
http://docs.python.org/lib/node535.html is not sufficient. Show how to
bind together cat(1) and tr(1). Cat should read from STDIN and pass the
same to STDOUT, being read by tr(1).
3. Nowhere is explained what this magic really does:
 output = p2.communicate()[0]
 I figured out the result is e.g. a string of '\n' separated lines.
 What is not clear whether at this very moment the processes get
 started and whether one can still write on input to them. It seems
 not, the p1.stdin has to be already populated with data.
Thanks.