proposition: Lemma onth_size s n x : onth s n = Some x -> n < size s.
proof: Proof. by case: onth_sizeP=>//->. Qed.
imports: Coq(ssreflect ssrbool ssrfun Eqdep) mathcomp(ssrnat seq eqtype choice fintype) pcm(options axioms) Coq(Classes.Morphisms Program.Basics Program.Tactics) Coq(Relations)