proposition: Lemma in_psupport s x : (x \in psupport s) = (s x != x).
proof: Proof. Proof using. by rewrite inE; apply/afix1P; case: eqP. 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