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plist-put modification by side effect

From: Dan Davison
Subject: plist-put modification by side effect
Date: Sat, 31 Jan 2009 16:06:30 -0500
User-agent: Mutt/1.5.18 (2008-05-17)

,----[C-h f plist-put]
| plist-put is a built-in function in `C source code'.
| (plist-put plist prop val)
| Change value in plist of prop to val.
| plist is a property list, which is a list of the form
| (PROP1 VALUE1 PROP2 VALUE2 ...).  prop is a symbol and val is any object.
| If prop is already a property on the list, its value is set to val,
| otherwise the new prop val pair is added.  The new plist is returned;
| use `(setq x (plist-put x prop val))' to be sure to use the new value.
| The plist is modified by side effects.

I don't get this. It says the plist is altered by side effects. So
what's with the "but just to be extra careful use (setq ...)"  advice?
I think I've seen a similar statemenmt in other docstrings, but I
can't remember which. Is there some subtlety that means you can't
actually rely on the side effect? Surely not?




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