code: func synchronizedWriters(targets ...io.Writer) []io.Writer {
  	mutex := &sync.Mutex{}
  	ret := make([]io.Writer, len(targets))
  	for i, target := range targets {
  		ret[i] = &synchronizedWriter{
  			Writer: target,
  			mutex:  mutex,
  		}
  	}
  	return ret
  }
docs: // synchronizedWriters takes a set of writers and returns wrappers that ensure
  // that only one write can be outstanding at a time across the whole set.