arc> (= a '(1 2)) (1 2) arc> (= (cddr a) a)
In ac.scm, ac-denil only considers tree like structures.
arc> (= a '(1 2)) (1 2) arc> (do (= (cddr a) a) t) t arc> (pop a) 1 arc> (pop a) 2 arc> (pop a) 1 arc> (pop a) 2
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