code: func Identity(sk crypto.PrivKey) Option {
  	return func(cfg *Config) error {
  		if cfg.PeerKey != nil {
  			return fmt.Errorf("cannot specify multiple identities")
  		}
  
  		cfg.PeerKey = sk
  		return nil
  	}
  }
docs: // Identity configures libp2p to use the given private key to identify itself.