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Used range
Sheet.LastRow and Sheet.LastColumn are 1-based and 0 for an empty sheet, and Sheet.UsedRange gives the enclosing A1-style range, so a sheet of unknown size can be walked without guessing where it ends.
Recalculation on load
The library does not evaluate formulas, so by default a workbook carrying them asks Excel to recalculate when it opens. That keeps written values from being shown stale, but Excel then prompts to save on close even when nothing was edited. Workbook.RecalculateOnLoad := False keeps the cached values exactly as written and drops the prompt.
Fixes
Reading a sheet no longer risks exhausting the regular expression engine on files that are mostly empty with styled self-closing cells. The shared-formula scan read <c r=../> as an opening tag and then searched for a closing tag that never came, once per self-closing cell. Thanks to @SteveNew for finding and fixing it.
