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(elisp)Constant Variables

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Variables that Never Change
===========================

   In Emacs Lisp, certain symbols normally evaluate to themselves.
These include `nil' and `t', as well as any symbol whose name starts
with `:' (these are called "keywords").  These symbols cannot be
rebound, nor can their values be changed.  Any attempt to set or bind
`nil' or `t' signals a `setting-constant' error.  The same is true for
a keyword (a symbol whose name starts with `:'), if it is interned in
the standard obarray, except that setting such a symbol to itself is
not an error.

     nil == 'nil
          => nil
     (setq nil 500)
     error--> Attempt to set constant symbol: nil

 - Function: keywordp object
     function returns `t' if OBJECT is a symbol whose name starts with
     `:', interned in the standard obarray, and returns `nil' otherwise.