I'm sure we all knew there had to be a good reason for &aux, but maybe we had trouble remembering what it might be?
(defstruct (delayed-iseq (:constructor iseq (start-or-end &optional (end 0 endp) (by 1) strict-direction? &aux (end? endp)))) "Delayed index sequence evaluation." start-or-end end end? by strict-direction?)
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