import Foundation

protocol GroupCollection {

   associatedtype SectionType
   associatedtype ObjectsType: Collection

   var key: SectionType { get }
   var objects: ObjectsType { get }
}

extension GroupCollection {

   var numberOfObjects: Int {
       return self.objects.count
   }
}

extension Collection where Iterator.Element: GroupCollection,
Index == Int, Iterator.Element.ObjectsType.Index == Int {

   typealias ObjectsType = Iterator.Element.ObjectsType

   subscript(path: IndexPath) -> ObjectsType.Iterator.Element {
       return self[path.section].objects[path.row]
   }
}

extension Collection where
   Iterator.Element: GroupCollection,
   Index == Int, Iterator.Element.ObjectsType.Index == Int,
Iterator.Element.ObjectsType.Iterator.Element: Equatable {

   private typealias ObjectType = Iterator.Element.ObjectsType.Iterator.Element

   private func indexPath(of object: ObjectType) -> IndexPath? {
       for (section, group) in self.enumerated() {