Alice and Bob can avoid a trusted arbiter if they meet in person, or if they somehow have fiat micropayments (so they
can have payment-channel like construction to fund the loan incrementally). Otherwise, some trusted arbiter is needed.
In any case, Alice and Bob are creating a box which contains Alice's collateral and also newly issued "loan tokens",
where 1 token = 1 unit of a loan currency. For example, assume that Alice borrowed 10,000 Swiss Franc, then the box
contains 10,000 loan tokens. Assume that market price now is about 10 Francs per 1 Ergo, then full collateral is about
1,000 Erg. However, the loan should be over-collateralized to avoid margin call in case of Ergo price decline.