proposition: Lemma onth_sizeN s : onth s (size s) = None.
proof: Proof. by apply/size_onthPn. 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)