code: @Override
      public List<ConfigMessage> init(Processor processor, ProcessorAction action, boolean predicate) {
          DataExpression expr = (DataExpression) getExpression();
  
          source = expr.getSource();
          key = expr.getKey();
  
          if (source == DataSource.Content) {
              index = Integer.parseInt(key);
          }
  
          return new ArrayList<>();
      }
docs: {@inheritDoc}