private QueryTree buildTree(OperatorNode<?> filterPart) {
        Preconditions.checkArgument(filterPart.getArguments().length == 2,
                                    "Expected 2 arguments to filter, got %s.",
                                    filterPart.getArguments().length);
        populateYqlSources(filterPart.getArgument(0));
        OperatorNode<ExpressionOperator> filterExpression = filterPart.getArgument(1);
        Item root = convertExpression(filterExpression);
        connectItems();
        userQuery = null;
        return new QueryTree(root);
    }
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