private static INotifyCollectionChanged getSource( INotifyCollectionChanged source, IEqualityComparer <TSourceItem> equalityComparer, int initialCapacity) => source.Grouping <TSourceItem, TSourceItem>(sourceItem => sourceItem, equalityComparer, initialCapacity);
private static INotifyCollectionChanged getSource( INotifyCollectionChanged source, IReadScalar <IEqualityComparer <TSourceItem> > equalityComparerScalar, int capacity) => source.Grouping <TSourceItem, TSourceItem>(sourceItem => sourceItem, equalityComparerScalar, capacity);