lens <TKey, TValue>(
this ImmutableDefaultDictionary <TKey, TValue> d,
TKey key)
=> d.lens[key];
ChainLens <TKey, TValue, Whole>(
this ImmutableDefaultDictionary <TKey, TValue> hole,
System.Func <ImmutableDefaultDictionary <TKey, TValue>, Whole> wrap)
=> new ImmutableDefaultDictionaryLens <TKey, TValue, Whole>(wrap: wrap, oldHole: hole);
