code: func (t *watcherTree) walk(fn walkFn) {
  	t.Tree.Walk(func(prefix string, val interface{}) bool {
  		watchers := val.([]*BufferWatcher)
  		for _, w := range watchers {
  			fn(w)
  		}
  		return false
  	})
  }
docs: // walk calls fn for every matching leaf of the tree