A borrowed money from B, the debt to be paid in installments. There were several receipts, each of which acknowledged the payment of a certain installment. B brought an action against A for installment not covered by receipts. A's defense was that he had paid the whole debt but that he could not show all the receipts for all the installments because there were instances when in his dealings with the creditor, no receipt was issued in his favor. Is A's contention sufficient to establish or prove the fact that the whole debt had been paid