private String fetchConditionWord(OperatorNode<ExpressionOperator> ast) {
        OperatorNode<ExpressionOperator> lhs = ast.getArgument(0);
        OperatorNode<ExpressionOperator> rhs = ast.getArgument(1);
        if (isNumber(lhs)) {
            assertFieldName(rhs);
            return getNumberAsString(lhs);
        }
        else if (isNumber(rhs)) {
            assertFieldName(lhs);
            return getNumberAsString(rhs);
        }
        else
            throw new IllegalArgumentException("Expected LITERAL/NEGATE 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