fact: Fixpoint ords x : pred (seq T) := fun y => match x , y with | [::] , [::] => false | [::] , t :: ts => true | x :: xs , y :: ys => if x == y then ords xs ys else ord x y | _ :: _ , [::] => false end.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path fintype) pcm(options)