/// <summary>
        /// 지정한 char 의 unicode string을 <paramref name="writer"/>에 쓴다.
        /// </summary>
        /// <param name="writer"></param>
        /// <param name="c"></param>
        public static void WriteCharAsUnicode(TextWriter writer, char c) {
            writer.ShouldNotBeNull("writer");

            var h1 = HexTextTool.IntToHex((c >> 12) & '\x000f');
            var h2 = HexTextTool.IntToHex((c >> 8) & '\x000f');
            var h3 = HexTextTool.IntToHex((c >> 4) & '\x000f');
            var h4 = HexTextTool.IntToHex(c & '\x000f');

            writer.Write('\\');
            writer.Write('u');
            writer.Write(h1);
            writer.Write(h2);
            writer.Write(h3);
            writer.Write(h4);
        }