private String fetchConditionIndex(OperatorNode<ExpressionOperator> ast) {
        OperatorNode<ExpressionOperator> lhs = ast.getArgument(0);
        OperatorNode<ExpressionOperator> rhs = ast.getArgument(1);
        if (isNumber(lhs))
            return getIndex(rhs);
        else if (isNumber(rhs))
            return getIndex(lhs);
        else
            throw new IllegalArgumentException("Expected LITERAL/VARREF and READ_FIELD/PROPREF, got " + lhs.getOperator() +
                                               " and " + rhs.getOperator() + ".");
    }
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