code: func (w *MapFieldWriter) clearTree(addr []string) {
  	prefix := strings.Join(addr, ".") + "."
  	for k := range w.result {
  		if strings.HasPrefix(k, prefix) {
  			delete(w.result, k)
  		}
  	}
  }
docs: // clearTree clears a field and any sub-fields of the given address out of the
  // map. This should be used to reset some kind of complex structures (namely
  // sets) before writing to make sure that any conflicting data is removed (for
  // example, if the set was previously written to the writer's layer).