code: func NewFilters(w *Whisper) *Filters {
  	return &Filters{
  		watchers:         make(map[string]*Filter),
  		topicMatcher:     make(map[TopicType]map[*Filter]struct{}),
  		allTopicsMatcher: make(map[*Filter]struct{}),
  		whisper:          w,
  	}
  }
docs: // NewFilters returns a newly created filter collection