5 “Infeasibility” is often a noun and “unfeasible”, as nan gray notes, is really an adjective herisson KeldericKelderic 47211 gold badge44 silver badges88 bronze badges ten You have to present proof. Long ago, among my instructors advocated "none is", so I exploit that (except at times I do not). Ngram https://reubene186wfm3.bloginder.com/profile