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From: Teemu Likonen <tlikonen@iki.fi>
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Subject: Is it safe to modify a property list directly with PLIST-PUT?
Date: Sun, 26 Jul 2009 19:38:40 +0300
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I use a list of property lists to store data. It's like this:

    (setq my-data '((:foo "one" :bar "two")
                    (:foo "three" :bar "four")))

Sometimes I need to modify the data and a command like this seems to
work:

    (plist-put (nth 1 my-data) :bar "New value")

That is, PLIST-PUT modifies the property list and variable MY-DATA
contains now the modified list:

    ((:foo "one" :bar "two")
     (:foo "three" :bar "New value"))

The question: Is this reliable? Is it guaranteed that it will always
modify the list correctly? If not, how would you suggest doing it
instead?

Common Lisp has so nice SETF macro...