private static void asString(Data data, StringBuilder b, String indent) {
        b.append(indent).append(data).append("\n");
        if (!(data instanceof DataList)) return;
        for (Data childData : ((DataList<? extends Data>) data).asList()) {
            asString(childData, b, indent.concat("    "));
        }
    }
Returns a data item as a recursively indented string
container-core/src/test/java/com/yahoo/processing/test/Responses.java