code: public void setOccurrence(String occurrence) throws CDKException {
          if (occurrence == null || occurrence.equals("")) {
              occurrence = ">0"; //revert to default
          } else {
              occurrence = occurrence.trim().replaceAll(" ", "");
              if (isValidOccurrenceSyntax(occurrence)) {
                  this.occurrence = occurrence;
              } else
                  throw new CDKException("Invalid occurence line: " + occurrence);
          }
      }
docs: Picky setter for occurrence fields. Validates user input to be conform
  the (Symyx) specification.
  @param occurrence occurence value