code: private void mapEngine(Map<String, Provider<? extends TemplateEngine>> map, Provider<? extends TemplateEngine> engine) {
          for (String type : engine.get().getContentType()) {
              map.put(type, engine);
          }
      }
docs: Map the engine to all the content types it supports.
  If any kind of overlap exists, a race condition occurs
  @param map
  @param engine