code: public CronDefinition instance() {
          final Set<CronConstraint> validations = new HashSet<>();
          validations.addAll(cronConstraints);
          final List<FieldDefinition> values = new ArrayList<>(fields.values());
          values.sort(FieldDefinition.createFieldDefinitionComparator());
          return new CronDefinition(values, validations, enforceStrictRanges, matchDayOfWeekAndDayOfMonth);
      }
docs: Creates a new CronDefinition instance with provided field definitions.
  
  @return returns CronDefinition instance, never null