One tricky thing to model neatly in payment APIs is that payment schemes indicate the roles of payer and payee in payment returns in different ways. E.g. for one particular scheme the payer and payee may be kept in the same position as in the initial payment (creditor of payment return is actually the one sending funds) whereas in another one they are switched (creditor is the one receiving funds in the return). I'd be curious to see how they are handling this case as it can be a real head-scratcher.