multiple-value-prog1 first-form {form}* → first-form-results

first-form — a form; evaluated as described below.

form — a form; evaluated as described below.

first-form-results — the values resulting from the evaluation of first-form.

multiple-value-prog1 evaluates first-form and saves all the values produced by that form. It then evaluates each form from left to right, discarding their values.

(setq temp '(1 2 3)) → (1 2 3) 
(multiple-value-prog1 
   (values-list temp) 
   (setq temp nil) 
   (values-list temp)) → 1, 2, 3

prog1