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);
