Cessions of reversionary rights in book debts

Cessions of book debts as security are commonplace instruments in the commercial world. What, however, is the precise legal standing of a cession to a creditor of the so-called reversionary rights or residual dominium of the cedent where the book debts as such have already been ceded to a first cessionary as security? The cedent is here endeavouring to cede as security to his second creditor, the residual value which may exist in his existing book debts after satisfaction of the debts due by him to his first creditor.