internal Enumerator(SortedTreeDictionary <TKey, TValue> .Enumerator enumerator)
 {
     _enumerator = enumerator;
 }
 internal ValueCollection(SortedTreeDictionary <TKey, TValue> dictionary)
 {
     Debug.Assert(dictionary != null, $"Assertion failed: {nameof(dictionary)} != null");
     _dictionary = dictionary;
 }