proposition: Lemma reindex_acts A a F : [acts A, on S | to] -> a \in A -> \big[op/idx]_(i in S) F i = \big[op/idx]_(i in S) F (to i a).
proof: Proof. by move=> nSA /(subsetP nSA); apply: reindex_astabs. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)