I find this example mildly infuriating/amusing:

    func Encrypt(message []byte) ([]byte, error) {
        var ciphertext []byte
        var encErr error
    
        secret.Do(func() {
            // ...
        })
        
        return ciphertext, encErr
    }

I get that the example is simplified (because what it should actually do is protect the long-term key, not the ephemeral one)... so, yeah, it's just a bad example.