public static IImmutableEnumerator <T> ToImmutableEnumerator <T>(this IEnumerator <T> e) => ImmutableEnumerator <T> .Make(e);
public ImmutableEnumeratorElement(U current, LazyList next, Last last)
{
this.current = current;
this.rest = new ImmutableEnumerator <U>(next, last);
this.hashCode = Equality.HashCode(current, rest);
}