public static Maybe <TSource> TryElementAt <TSource>(this List <TSource> source, int index)
 => ReadOnlyList.TryElementAt <List <TSource>, TSource>(source, index);
 public static Maybe <TSource> TryElementAt <TSource>(this ImmutableArray <TSource> source, int index)
 => ReadOnlyList.TryElementAt <ImmutableArray <TSource>, TSource>(source, index);