code: public void registerTemplateEngine(Class<? extends TemplateEngine> engineClass) {
          if (templateEngine != null) {
              log.debug("Template engine already registered, ignoring '{}'", engineClass.getName());
              return;
          }
  
          try {
              TemplateEngine engine = engineClass.newInstance();
              setTemplateEngine(engine);
          } catch (Exception e) {
              throw new PippoRuntimeException(e, "Failed to instantiate '{}'", engineClass.getName());
          }
      }
docs: Registers a template engine if no other engine has been registered.
  
  @param engineClass