code: private static Set<Long> getFeedItemIdsFromArgument(Function function) {
      if (function.getLhsOperand().length == 1
          && function.getLhsOperand(0) instanceof RequestContextOperand) {
        RequestContextOperand requestContextOperand =
            (RequestContextOperand) function.getLhsOperand(0);
        if (RequestContextOperandContextType.FEED_ITEM_ID.equals(
            requestContextOperand.getContextType())
            && FunctionOperator.IN.equals(function.getOperator())) {
          return Arrays.stream(function.getRhsOperand())
              .filter(ConstantOperand.class::isInstance)
              .map(argument -> ((ConstantOperand) argument).getLongValue())
              .collect(Collectors.toSet());
        }
      }
  
      return new HashSet<>();
    }
docs: Gets the set of feed item IDs from the function if it is of the form:
  <code>IN(FEED_ITEM_ID,{xxx,xxx})</code>. Otherwise, returns an empty set.