code: @Override
      public IssueResponse issue(String CorpNum, MgtKeyType KeyType, String MgtKey,
                            String Memo) throws PopbillException {
          return issue(CorpNum, KeyType, MgtKey, Memo, null);
      }
docs: /* (non-Javadoc)
  @see com.popbill.api.TaxinvoiceService#issue(java.lang.String, com.popbill.api.taxinvoice.MgtKeyType, java.lang.String, java.lang.String)