code: public Response assignMgtKey(String corpNum, MgtKeyType keyType, String itemKey,
                                   String mgtKey) throws PopbillException {
  
          return assignMgtKey(corpNum, keyType, itemKey, mgtKey, null);
      }
docs: /*
  (non-Javadoc)
  @see com.popbill.api.TaxinvoiceService#assignMgtKey(java.lang.String, com.popbill.api.taxinvoice.MgtKeyType, java.lang.String, java.lang.String)