private static XContentBuilder generate(List<KnapsackState> values) throws IOException {
   XContentBuilder builder = jsonBuilder();
   builder.startObject();
   builder.startArray("array");
   for (KnapsackState value : values) {
     value.toXContent(builder, EMPTY_PARAMS);
   }
   builder.endArray();
   builder.endObject();
   return builder;
 }