code: public CRLNumberExtension getCRLNumberExtension() throws IOException {
          Object obj = getExtension(PKIXExtensions.CRLNumber_Id);
          return (CRLNumberExtension)obj;
      }
docs: return the CRLNumberExtension, if any.
  
  @returns CRLNumberExtension or null (if no such extension)
  @throws IOException on error