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 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 symbol whose name starts with `:', except that you are allowed to set such a symbol to itself.

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

Variable: keyword-symbols-constant-flag

If this variable is nil, you are allowed to set and bind symbols whose names start with `:' as you wish. This is to make it possible to run old Lisp programs which do that.

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