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In the execution this is an array access, but the logical definition is just a thin wrapper for nth:
Function:
(defun 4vecs$ai (stobjs::i 4vecarr$a) (declare (xargs :guard (and (4vecarr$ap 4vecarr$a) (integerp stobjs::i) (<= 0 stobjs::i) (< stobjs::i (4vecs$a-length 4vecarr$a))))) (ec-call (4vec-fix$inline (ec-call (nth stobjs::i 4vecarr$a)))))