private void populateYqlSummaryFields(List<OperatorNode<ProjectOperator>> fields) {
        yqlSummaryFields.clear();
        for (OperatorNode<ProjectOperator> field : fields) {
            assertHasOperator(field, ProjectOperator.FIELD);
            yqlSummaryFields.add(field.getArgument(1, String.class));
        }
    }
The YQL query language.

This class <em>must</em> be kept in lockstep with {@link VespaSerializer}.
Adding anything here will usually require a corresponding addition in
VespaSerializer.

@author Steinar Knutsen
@author Stian Kristoffersen
@author Simon Thoresen Hult
container-search/src/main/java/com/yahoo/search/yql/YqlParser.java