code: func fillOptions(opts *Options) *Options {
  	if opts == nil {
  		opts = &Options{}
  	}
  	// copy and reassign the options
  	nopts := *opts
  	if nopts.LogOutput == nil {
  		nopts.LogOutput = os.Stderr
  	}
  	return &nopts
  }
docs: // fillOptions fills in default options