code: public Page serviceAt(String uri, Class<?> pageClass) {
      // Handle subpaths, registering each as a separate instance of the page
      // tuple.
      for (Method method : pageClass.getDeclaredMethods()) {
        if (method.isAnnotationPresent(At.class)) {
  
          // This is a subpath expression.
          At at = method.getAnnotation(At.class);
          String subpath = at.value();
  
          // Validate subpath
          if (!subpath.startsWith("/") || subpath.isEmpty() || subpath.length() == 1) {
            throw new IllegalArgumentException(String.format(
                "Subpath At(\"%s\") on %s.%s() must begin with a \"/\" and must not be empty",
                subpath, pageClass.getName(), method.getName()));
          }
  
          subpath = uri + subpath;
  
          // Register as headless web service.
          doAt(subpath, pageClass, true);
        }
      }
  
      return doAt(uri, pageClass, true);
    }
docs: Page registration (internal) APIs