fact: Definition cast_perm m n (eq_mn : m = n) (s : 'S_m) := let: erefl in _ = n := eq_mn return 'S_n in s.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)