docs: def init_app(self, app):
          """Initialize extension to the given application.
  
          Extension will be registered to `app.extensions` with lower classname
          as key and instance as value.
  
          :param app: Flask application.
          """
          self.init_extension(app)
  
          if not hasattr(app, 'extensions'):
              app.extensions = {}
          classname = self.__class__.__name__
          extname = classname.replace('Flask', '').lower()
          app.extensions[extname] = self