proposition: Theorem third_isog : (G / H / (K / H)) \isog (G / K).
proof: Proof. by case: third_isom => f inj_f im_f; apply/isogP; exists f; rewrite ?im_f. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)