Inputs to computeEntireDebt: the draw, fee, rate, and elapsed time.
Requested draw (MUSD the borrower receives).
Seconds since the principal was last touched (0 = at open).
Borrowing fee for the draw (caller reads getBorrowingFee; governable).
getBorrowingFee
The Trove's interest rate, in basis points.
Inputs to computeEntireDebt: the draw, fee, rate, and elapsed time.