docs: def filter_simple(filter):
      """
      A simpler version of :func:`filter`. This instead assumes the decorated
      function is already a valid filter function, and uses it directly.
      """
      def function_getter(function):
          if isinstance(function, Filter):
              function.add_filter(filter)
  
              return function
          else:
              return Filter(
                  filter=filter,
                  callback=function,
              )
  
      return function_getter