/// <inheritdoc/>
 public IEnumerator <KeyValuePair <TKey, BigInteger> > GetEnumerator() =>
 _underlying.Select(x => new KeyValuePair <TKey, BigInteger>(x.Key, x.Value.Value)).GetEnumerator();
 public IEnumerator <KeyValuePair <TKey, IImmutableSet <TValue> > > GetEnumerator() =>
 Underlying.Select(x => new KeyValuePair <TKey, IImmutableSet <TValue> >(x.Key, x.Value.Elements)).GetEnumerator();