The answer is based on the fact that 'pop operates on a "place". In other words, it is expecting as input something that can be assigned to.
To see an example:
arc> (= a '(1 2 3 4))
(1 2 3 4)
arc> (pop cdr.a)
2
arc> a
(1 3 4)
You'll notice that not only did pop return 2, it also removed it from a. This functionality is provided to pop by using the macro 'setforms, which relies on a table 'setter that maps forms to functions that can assign to them.
So cdr has a setter defined by
(defset cdr (x)
(w/uniq g
(list (list g x)
`(cdr ,g)
`(fn (val) (scdr ,g val)))))
but nthcdr doesn't currently have a setter defined for it.
So, if you want nthcdr to work with pop and similar destructive functions, you will need to define a setter for it.
