code: public BigInteger getCRLNumber() throws IOException {
          CRLNumberExtension numExt = getCRLNumberExtension();
          if (numExt != null) {
              BigInteger num = numExt.get(CRLNumberExtension.NUMBER);
              return num;
          } else {
              return null;
          }
      }
docs: return the CRL number from the CRLNumberExtension, if any.
  
  @returns number or null (if no such extension)
  @throws IOException on error