code: public BigInteger get(String name) throws IOException {
          if (name.equalsIgnoreCase(NUMBER)) {
              if (crlNumber == null) return null;
              else return crlNumber;
          } else {
            throw new IOException("Attribute name not recognized by"
                                  + " CertAttrSet:" + extensionName + ".");
          }
      }
docs: Get the attribute value.