16.2 Variables That Never Change

In SXEmacs Lisp, some symbols always evaluate to themselves: the two special symbols nil and t, as well as keyword symbols, that is, symbols whose name begins with the character ‘:’. These symbols cannot be rebound, nor can their value cells be changed. An attempt to change the value of nil or t signals a setting-constant error.

nil ≡ 'nil
     ⇒ nil

(setq nil 500)
error→ Attempt to set constant symbol: nil