private static boolean isIndexOnLeftHandSide(OperatorNode<ExpressionOperator> ast) {
        OperatorNode<?> node =  ast.getArgument(0, OperatorNode.class);
        return node.getOperator() == ExpressionOperator.READ_FIELD || node.getOperator() == ExpressionOperator.PROPREF;
    }
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