PhD student solves 2,500-year-old Sanskrit problem

Ah more details from a better article:

https://www.cam.ac.uk/stories/solving-grammars-greatest-puzz...

Pāṇini taught a metarule – termed by Rajpopat ‘1.4.2 vipratiṣedhe paraṁ kāryam’ – to help us decide which rule should be applied in the event of ‘rule conflict’ but for the last 2,500 years, scholars have misinterpreted this metarule meaning that they often ended up with a grammatically incorrect result.

Curious what the sutra was and was there no varttika or mahabhashya on it?