private boolean isNumber(OperatorNode<ExpressionOperator> ast) {
        return ast.getOperator() == ExpressionOperator.NEGATE ||
               ast.getOperator() == ExpressionOperator.LITERAL || ast.getOperator() == ExpressionOperator.VARREF;
    }
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