$$In the absence of market imperfections and swap externalities, swaps are a zero$-$sum game such that the QSD cannot be exploited for economic gain, that is$,$ the QSD cannot be arbitraged.$$ Considering this statement, outline and explain Wall$$s agency$-$theoretic explanation $($Wall$,$ L$.$D$.,1989,$ Interest Rate Swaps in an Agency Theoretic Model With Uncertain Interest Rates. Journal of Banking and Finance $13,261270)$ and one other explanation of your choice for why the QSD exists.