proposition: Lemma trivIset_in : trivIset C -> x \in C -> trivIset (x |: C).
proof: Proof. move/trivIsetP => HC Hx. apply/trivIsetP => y z. rewrite !in_setU1. by move/orP => [/eqP -> | ?]; move/orP => [/eqP -> | ?]; apply HC. Qed.
imports: mathcomp(all_ssreflect ssralg fingroup zmodp poly ssrnum) mathcomp(matrix perm) ssr_ext ssralg_ext fdist