Comment by DonaldFisk

I was puzzled by this: (rplacd (quote tt) (quote ...)) and later (print (cdr (quote tt))).

But it appears to be valid in PDP-1 Lisp (though not in either Lisp 1.5 or modern Lisps). From https://s3data.computerhistory.org/pdp-1/DEC.pdp_1.1964.1026...

"Doing a CDR of an atom is permissible and will get the atom's property list. Doing a CAR of an atom may very easily wreck the system."