public int getExternalSheetIndex(String workbookName, String firstSheetName, String lastSheetName) {
int externalBookIndex = getExternalWorkbookIndex(workbookName);
if (externalBookIndex == -1) {
throw new RuntimeException("No external workbook with name '" + workbookName + "'");
}
SupBookRecord ebrTarget = _externalBookBlocks[externalBookIndex].getExternalBookRecord();
int firstSheetIndex = getSheetIndex(ebrTarget.getSheetNames(), firstSheetName);
int lastSheetIndex = getSheetIndex(ebrTarget.getSheetNames(), lastSheetName);
// Find or add the external sheet record definition for this
int result = _externSheetRecord.getRefIxForSheet(externalBookIndex, firstSheetIndex, lastSheetIndex);
if (result < 0) {
result = _externSheetRecord.addRef(externalBookIndex, firstSheetIndex, lastSheetIndex);