code: public Invoice getOriginalInvoice() {
          if (originalInvoice != null && originalInvoice.getHref() != null && !originalInvoice.getHref().isEmpty()) {
              originalInvoice = fetch(originalInvoice, Invoice.class);
          }
          return originalInvoice;
      }
docs: Fetches the original invoice if the href is populated, otherwise return the current original invoice.
  
  @return fully loaded original invoice