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