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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 lhss$ai (stobjs::i lhsarr$a) (declare (xargs :guard (and (lhsarr$ap lhsarr$a) (integerp stobjs::i) (<= 0 stobjs::i) (< stobjs::i (lhss$a-length lhsarr$a))))) (ec-call (lhs-fix$inline (ec-call (nth stobjs::i lhsarr$a)))))