proposition: Lemma head_nilp (x : A) xs : x \notin xs -> head x xs = x -> nilp xs.
proof: Proof. elim: xs=>[|y ys IH] //= H1 H2. by rewrite H2 inE eqxx /= in H1. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat seq path eqtype choice bigop) pcm(options prelude pred)