code: func (s *ServerConfig) AddHostKey(key Signer) {
  	for i, k := range s.hostKeys {
  		if k.PublicKey().Type() == key.PublicKey().Type() {
  			s.hostKeys[i] = key
  			return
  		}
  	}
  
  	s.hostKeys = append(s.hostKeys, key)
  }
docs: // AddHostKey adds a private key as a host key. If an existing host
  // key exists with the same algorithm, it is overwritten. Each server
  // config must have at least one host key.