private ExcelSheet getExcelSheet(String sheet){
if(BLANK_SHEET_DEF.equalsIgnoreCase(sheet)) return BLANK_SHEET;
ExcelSheet sheetDefinition = sheetDefinitions.get(sheet);
if(sheetDefinition == null)
throw new RuntimeException("No sheet defintion found with name: " + sheet);
return sheetDefinition.cloneSheet();
}
}