docs: def register_filter(self, filter_name, filter_ref, force=False):
          """
          Add/register one filter.
  
          Args:
              filter_name (str): Filter name used inside :program:`Jinja2` tags.
              filter_ref: Reference to the filter itself, i.e. the corresponding :program:`Python` function.
              force (bool): If set to ``True``, forces the registration of a filter no matter if it already exists or not.
  
          Note:
              The list of user added/registered filters can be retrieve with :mth:`registered_filters_list`
          """
          if not force and (filter_name in self.filters_list()):
              self.log_warning("Extension %s already exist, ignore redefinition." % ext_in)
              return
  
          self.__jinja2_environment.filters[filter_name] = filter_ref