I want to know the answer to the following question:
Nagarjuna's main contribution was the exposition of the concept of what?
All the background I'm provided with is that "Nagarjuna's primary contribution to Buddhist philosophy was the systematic exposition of the concept of śūnyatā, or "emptiness", widely attested in the Prajñāpāramitā sutras that emerged in his era.". Is that enough to answer the question?