private ExpressionNode transformFeature(ReferenceNode node, TransformContext context) {
        if ( ! node.getArguments().isEmpty() && ! isSimpleFeature(node.reference()))
            return transformArguments(node, context);
        else
            return transformConstantReference(node, context);
    }
Returns true if the given reference is an attribute, constant or query feature
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/transform/ConstantDereferencer.java