for reference see 7 & BD16 in http://unicode.org/reports/tr9/
function _symmetricSwap(charArray, levels, params) {
      if (params.hiLevel !== 0 && _isSymmetricSwapping) {
        for (var i = 0, index; i < charArray.length; i++) {
          if (levels[i] === 1) {
            index = _SWAP_TABLE.indexOf(charArray[i]);

            if (index >= 0) {
              charArray[i] = _SWAP_TABLE[index + 1];
            }
          }
        }
      }
    }