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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 bits$ai (stobjs::i bitarr$a) (declare (xargs :guard (and (bitarr$ap bitarr$a) (integerp stobjs::i) (<= 0 stobjs::i) (< stobjs::i (bits$a-length bitarr$a))))) (ec-call (bfix (ec-call (nth stobjs::i bitarr$a)))))