T. Lucreti Cari De Rerum Natura Libri Sex.
Edited with Introduction and Commentary by William Ellery Leonard, Stanley Barney Smith (1942; rpt. Madison: University of Wisconsin Press, 1970), p. 204 (on 1.50: quod superest, [ut] uacuas auris animumque sagacem):
[ut]. The erroneous intrusion seems to have arisen from a repetition and transposition of the final letter of superest and the initial letter of uacuas.
An unlikely explanation. A simpler and more likely explanation is that superest is often followed in Latin by ut.