Fixed.
fpspreadsheet now supports "3d references", i.e. cell addresses an other sheets of the same workbook. Cell ranges can be in a single sheet or in a range of sheets.
The syntax is that of Excel, e.g. Sheet3!A1 (single cell), Sheet3!A1:B3 (cell range in single sheet), Sheet3:Sheet6!A1 (the same cell in a range of sheets, Sheet3:Sheet6!A1:B3 (the same cell range in a range of sheets). The syntax of LibreOffice Calc (e.g. Sheet3.A1:Sheet6.B3 is supported internally for reading and writing .ods files, but is not allowed in the TsWorksheet's method WriteFormula. Maybe this will change - I don't know...
The readers and writers of the formats Excel 95, 97 (BIFF5 and BIFF8, .xls) and 2007+ (.xlsx), as well as OpenDocument (.ods) are supporting the new feature. Excel 2 is not supported because it serves only a single worksheet anyway.
References to other files still are not allowed. I don't know whether I will implement this. Use the workaround with the workbook option [boIgnoreFormulas] if you must read such files and write them back -- but you must be aware that the formula engine is disabled with this option.
All provided unit tests are passed, but of course I cannot exclude that some bugs are still hidden somewhere. So please test.
The new feature has consequences on inserting/deleteing of rows and columns - the required adaptions are have not been implemented yet. Moreover I expect some speed pentaly for large workbooks with many sheets and formulas when the workbook option boAutoCalc is set because now the entire workbook has to be recalculated with every change of any cell - the old version only had to recalculate the sheet containing the changed cell.