public static ReadOnlySpan <char> Slice(this ReadOnlySpan <char> original, ReadOnlySpanCursor start)
 {
     return(original.Slice(Math.Min(original.Length, start(original))));
 }
        public static ReadOnlySpan <char> Slice(this ReadOnlySpan <char> original, ReadOnlySpanCursor start, ReadOnlySpanCursor end)
        {
            var startIndex = start(original);

            return(original.Slice(startIndex, end(original) - startIndex));
        }