fact: Fixpoint onth (s : seq A) n : option A := if s is x::sx then if n is nx.+1 then onth sx nx else Some x else None.
imports: Coq(ssreflect ssrbool ssrfun Eqdep) mathcomp(ssrnat seq eqtype choice fintype) pcm(options axioms) Coq(Classes.Morphisms Program.Basics Program.Tactics) Coq(Relations)