an object.

implementation-dependent.

The variables +, ++, and +++ are maintained by the Lisp read-eval-print loop to save forms that were recently evaluated.

The value of + is the last form that was evaluated, the value of ++ is the previous value of +, and the value of +++ is the previous value of ++.

(+ 0 1) → 1 
(- 4 2) → 2 
(/ 9 3) → 3 
(list + ++ +++) → ((/ 9 3) (- 4 2) (+ 0 1)) 
(setq a 1 b 2 c 3 d (list a b c)) → (1 2 3) 
(setq a 4 b 5 c 6 d (list a b c)) → (4 5 6) 
(list a b c) → (4 5 6) 
(eval +++) → (1 2 3) 
#.`(,@++ d) → (1 2 3 (1 2 3))

Lisp read-eval-print loop.

- (variable), * (variable), / (variable), Section 25.1.1 (Top level loop)