public protocol _ModSubgroupRelation: EquivalenceRelation where Base == Sub.Super { associatedtype Sub: Subgroup } public struct ModSubgroupRelation<H: Subgroup>: _ModSubgroupRelation { public typealias Base = H.Super public typealias Sub = H ... } public typealias QuotientGroup<G, H: Subgroup> = QuotientSet<G, ModSubgroupRelation<H>> where G == H.Super extension QuotientGroup: Group where Rel: _ModSubgroupRelation, X == Rel.Base { ... }
できたっぽいです！