private OperatorNode<?> fetchSummaryFields(OperatorNode<?> ast) {
        if (ast.getOperator() != SequenceOperator.PROJECT) return ast;
        Preconditions.checkArgument(ast.getArguments().length == 2,
                                   "Expected 2 arguments to PROJECT, got %s.",
                                   ast.getArguments().length);
        populateYqlSummaryFields(ast.getArgument(1));
        return ast.getArgument(0);
    }
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