[ENG]
It is an archaic feature, thus found mainly in old writers (especially in poetry). One famous example can be read in the first lines of Cicero's De senectute: levasso. It is often translated by the future perfect, so, for instance, levasso = levavero.
[LAT]
Est enim forma vetusta, potissimum apud veteres scriptores reperta (præsertim in re poëtica). Unum exemplum præclarum plane legitur jam initio libri illius a Cicerone conscripti, c.n. De senectute: levasso. Sæpenumero convertitur per futurum perfectum, e.g.: levasso = levavero.