a proper list.
The variables /, //, and /// are maintained by the Lisp read-eval-print loop to save the values of results that were printed at the end of the loop.
The value of / is a list of the most recent values that were printed, the value of // is the previous value of /, and the value of /// is the previous value of //.
The values of /, //, and /// are updated immediately prior to printing the return value of a top-level form by the Lisp read-eval-print loop. If the evaluation of such a form is aborted prior to its normal return, the values of /, //, and /// are not updated.
(floor 22 7) ⇒ 3, 1
(+ (* (car /) 7) (cadr /)) ⇒ 22