One engine, six editions
A 23-sheet ledger that rebuilds a year of accounts from bank statements and cash notes, issued in six visual editions of one identical engine.
formulas in each of the six editions, identical to the formula. Only the look changes - which means a design can be chosen without the risk that the file is different underneath.
What was done
One build engine was written, then six look dictionaries - so all six files carry the same 8,148 formulas, the same sheets and the same row geometry. What differs is the palette, the typeface, the row heights and whether the gridlines are part of the design.
The reason is one thing: a forked build script diverges on the first bug fix that only lands in one copy. Six files generated from one engine cannot do that.
Every edition carries its own twelve checks, each computing the same figure down two different paths and printing the difference. Twelve of twelve green in real Excel, zero error cells.