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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