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place—a place, the value of which is a list (possibly, but necessarily, a dotted list or circular list).
element—an object (the car of the contents of place).
pop reads the value of place,
remembers the car of the list which was retrieved,
writes the cdr of the list back into the place,
and finally yields the car of the originally retrieved list.
For information about the evaluation of subforms of place, see Section 188.8.131.52 (Evaluation of Subforms to Places).
(setq stack '(a b c)) → (A B C) (pop stack) → A stack → (B C) (setq llst '((1 2 3 4))) → ((1 2 3 4)) (pop (car llst)) → 1 llst → ((2 3 4))
The contents of place are modified.
push, pushnew, Section 5.1 (Generalized Reference)
The effect of
) is roughly equivalent to
(prog1 (car place) (setf place (cdr place)))
except that the latter would evaluate any subforms of place
three times, while
pop evaluates them only once.