private void filter(Result result) {
        if (result.getQuery().properties().getBoolean(FIELD_FILTER_DISABLE)) return;
        if (result.getQuery().getPresentation().getSummaryFields().isEmpty()) return;

        for (Iterator<Hit> i = result.hits().unorderedDeepIterator(); i.hasNext();) {
            Hit h = i.next();
            if (h.isMeta()) continue;
            for (var fields = h.fieldIterator(); fields.hasNext(); ) {
                var field = fields.next();
                if ( ! result.getQuery().getPresentation().getSummaryFields().contains(field.getKey()) &&
                     ! syntheticFields.contains(field.getKey()))
                {
                    fields.remove();
                }
            }
        }
    }
Fields that should be kept even if not explicitly requested
container-search/src/main/java/com/yahoo/search/yql/FieldFilter.java