Section Commutator_properties.
Variable gT : finGroupType.
Implicit Type (rT : finGroupType) (A B C : {set gT}) (D G H K : {group gT}).

Lemma quotient_cents2 A B K :
A \subset 'N(K) -> B \subset 'N(K) ->
(A / K \subset 'C(B / K)) = ([~: A, B] \subset K).