* @return the shifted formula if the formula was changed,
* <code>null</code> if the formula wasn't modified
*/
private static String shiftFormula(XSSFRow row, String formula, FormulaShifter shifter) {
XSSFSheet sheet = row.getSheet();
XSSFWorkbook wb = sheet.getWorkbook();
int sheetIndex = wb.getSheetIndex(sheet);
XSSFEvaluationWorkbook fpb = XSSFEvaluationWorkbook.create(wb);
try {
Ptg[] ptgs = FormulaParser.parse(formula, fpb, FormulaType.CELL, sheetIndex);
String shiftedFmla = null;