public static bool Contains <TKey, TValue>(this SortedDictionary <TKey, TValue> .KeyCollection source, TKey value, IEqualityComparer <TKey>?comparer)
 => ValueReadOnlyCollectionExtensions.Contains <ValueWrapper <TKey, TValue>, SortedDictionary <TKey, TValue> .KeyCollection.Enumerator, TKey>(new ValueWrapper <TKey, TValue>(source), value, comparer);
 public static bool Contains <TKey, TValue>(this Dictionary <TKey, TValue> source, KeyValuePair <TKey, TValue> value, IEqualityComparer <KeyValuePair <TKey, TValue> >?comparer)
     where TKey : notnull
 => ValueReadOnlyCollectionExtensions.Contains <ValueWrapper <TKey, TValue>, Dictionary <TKey, TValue> .Enumerator, KeyValuePair <TKey, TValue> >(new ValueWrapper <TKey, TValue>(source), value, comparer);
 public static bool Contains <TSource>(this ImmutableHashSet <TSource> source, TSource value, IEqualityComparer <TSource>?comparer = default)
 => ValueReadOnlyCollectionExtensions.Contains <ValueWrapper <TSource>, ImmutableHashSet <TSource> .Enumerator, TSource>(new ValueWrapper <TSource>(source), value, comparer);