public void TestEnumerator()
{
ImmutableSortedTreeSet <int> .Builder set = ImmutableSortedTreeSet.CreateBuilder <int>();
ImmutableSortedTreeSet <int> .Enumerator enumerator = set.GetEnumerator();
Assert.Equal(0, enumerator.Current);
Assert.False(enumerator.MoveNext());
Assert.Equal(0, enumerator.Current);
Assert.False(enumerator.MoveNext());
Assert.Equal(0, enumerator.Current);
// Adding an item to the list invalidates it, but Current is still unchecked
CollectionAssert.EnumeratorInvalidated(set, () => set.Add(1));
Assert.Throws <InvalidOperationException>(() => enumerator.MoveNext());
Assert.Equal(0, enumerator.Current);
enumerator = set.GetEnumerator();
Assert.Equal(0, enumerator.Current);
Assert.True(enumerator.MoveNext());
Assert.Equal(1, enumerator.Current);
// Reset has no effect due to boxing the value type
((IEnumerator <int>)enumerator).Reset();
Assert.Equal(1, enumerator.Current);
Assert.False(enumerator.MoveNext());
Assert.Equal(1, enumerator.Current);
}