Powerful pattern enabled by transient storage. The user calls NonCustodialFlashLoans#startLoan, and control is handed back to them using the IStartCallback#start callback. The user can then borrow as much of any token they want, from any user that has sent approval, within this callback by calling NonCustodialFlashLoans#borrow. The contract uses transient storage to track what has been borrowed and check the tokens are returned at the end. The SLOAD/SSTORE’s make this pattern gas-infeasible without transient storage. Credit: @sendmoodz for this idea