code: def before_init_app(self, app: FlaskUnchained):
          
          from .templates import (UnchainedJinjaEnvironment,
                                  UnchainedJinjaLoader)
          app.jinja_environment = UnchainedJinjaEnvironment
          app.jinja_options = {**app.jinja_options,
                               'loader': UnchainedJinjaLoader(app)}
          app.jinja_env.globals['url_for'] = url_for
  
          for name in ['string', 'str']:
              app.url_map.converters[name] = StringConverter
docs: Configure the Jinja environment and template loader.