public static void AddNested <TOuter, TInner>(
     this IEnumerable <TOuter> outerItems,
     Func <TOuter, ICollection <TInner> > innerCollectionSelector,
     TOuter outerItem,
     TInner innerItemToAdd
     )
 {
     outerItems.AddNested(innerCollectionSelector, outerItem, _ => innerItemToAdd);
 }
 public static void AddNested <TOuter, TInner>(
     this IEnumerable <TOuter> outerItems,
     Func <TOuter, ICollection <TInner> > innerCollectionSelector,
     TOuter outerItem,
     Func <TOuter, TInner> innerProducer
     )
 {
     outerItems.AddNested(innerCollectionSelector, i => i.Equals(outerItem), innerProducer);
 }