code: public Invoices getOriginalInvoices(final String invoiceId) {
          return doGET(Invoices.INVOICES_RESOURCE + "/" + invoiceId + "/original_invoices",
                      Invoices.class, new QueryParams());
      }
docs: Lookup an invoice's original invoices (e.g. a refund invoice has original_invoices)
  <p>
  Returns the invoice's original invoices
  
  @param invoiceId the invoice id
  @return the original invoices associated with this invoice on success. Throws RecurlyAPIError if not found