code: func (w *Writer) Attach(logger ...*log.Logger) {
  	if len(logger) == 0 {
  		logger = []*log.Logger{nil}
  	}
  	for _, l := range logger {
  		lw := newLogWriter(w, l)
  		if l == nil {
  			log.SetOutput(lw)
  		} else {
  			l.SetOutput(lw)
  		}
  	}
  }
docs: // Attach sets this writer as the output destination
  // for the specified logger. If the logger is not specified,
  // then this writer attaches to the log package 'standard' logger.
  //
  // This method calls SetOutput for the specified logger
  // (or the standard logger) to set its output writer.