proposition: Lemma permPr s1 s2 : reflect (perm_eqr s1 s2) (perm_eq s1 s2).
proof: Proof. by apply/(iffP idP) => [/permPl eq12 s3| <- //]; rewrite !(perm_sym s3) eq12. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)