fact: CoInductive size_spec : nat -> seq A -> Type := | SizeNil : size_spec 0 [::] | SizeCons x s : size_spec (size s).+1 (x :: s).
imports: mathcomp(all_ssreflect ssralg fingroup zmodp poly ssrnum) mathcomp(matrix perm) ssr_ext ssralg_ext fdist