fact: Definition ker_cprod_by of isom 'Z(H) 'Z(K) gz := [set xy | let: (x, y) := xy in (x \in 'Z(H)) && (y == (gz x)^-1)].
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient action gproduct gfunctor) mathcomp(cyclic)