proposition: Lemma psupport_stable s x : (x \in psupport s) = (s x \in psupport s).
proof: Proof. Proof using. rewrite !in_psupport; congr negb; apply/idP/idP => [/eqP -> // | /eqP H]. by rewrite -[s x](permK s) H permK. Qed.
imports: mathcomp.ssreflect.ssreflect mathcomp(ssrfun ssrbool eqtype ssrnat seq fintype) mathcomp(tuple path bigop finset div) mathcomp(fingroup perm action ssralg) tools permcomp