Virtual table function calls

I’ve written a virtual table to view spreadsheet data inside Excel workbook (.xlsx) files as SQlite tables. I want to implement an SQL function, attr(), which the virtual table overrides to give access to the raw attributes of the cell being returned. I’m looking for a robust way to find the actual column refered to by an overriden function, not just the value returned by the xColumn call.

Example:
   sqlite> .load xlsxvtab.so                                         # creates xlsxvtab module and attr() function
   sqlite> create virtual table summary using xlsxvtab(‘expenses.xlsx’,’sheet1’,’F20’, ‘H32’);
   sqlite> select F,G,H,attr(H,3) from summary;           # show formula used to calculate column H.

The issue is that attr gets called with 2 values and I need to divine that the first value came from column H of the current row of the cursor opened by this select statement. The hack I’m using now is to give the values returned by the xColumn method a subtype equal to the column number. In the attr() function I retrieve the subtype and re-fetch that column from the current row of the last cursor open on that table.

Various pitfalls with this technique:
    - Subtype numbers are limited to the range 0-255.
    - Assumes all functions called before xNext() method called again.
    - Complex queries with multiple cursors?

Any suggestions?
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