[specialist]

Ya. You reminded me that any Excel clone (or interop) has to reproduce Excel's bugs too.

No fun.

Ages ago, I was buddies with a guy (Dale?) who reverse engineered DWG, which I think became the code dump for the Open DWG Alliance, which may be this group today https://en.wikipedia.org/wiki/Open_Design_Alliance, though I don't see McNeel & Assoc listed (Rhino3D).

Dale worked for a graphics card company, Appian?, writing drivers and file viewers. He'd reversed DWG to write a wicked fast viewer. Just like those guys from Atlanta (don't remember their names).

I now dimly recall that Dale's team had to also reproduce AutoCAD's rendering bugs.