/* ----------------------------------------------------------------- */
        ///
        /// AddEncryptionMethod
        ///
        /// <summary>
        /// 暗号化方式を追加します。
        /// </summary>
        ///
        /* ----------------------------------------------------------------- */
        private void AddEncryptionMethod(ZipOption zo)
        {
            var value = zo.EncryptionMethod;

            if (value == EncryptionMethod.Default)
            {
                return;
            }
            Add("em", PropVariant.Create(value.ToString()));
        }
        /* ----------------------------------------------------------------- */
        ///
        /// AddCompressionMethod
        ///
        /// <summary>
        /// 圧縮方式を追加します。
        /// </summary>
        ///
        /* ----------------------------------------------------------------- */
        private void AddCompressionMethod(ZipOption zo)
        {
            var value = zo.CompressionMethod;

            if (!SupportedMethods.Contains(value))
            {
                return;
            }
            Add("m", PropVariant.Create(value.ToString()));
        }