code: func (srv *Server) AddHostKey(key Signer) {
  	// these are later added via AddHostKey on ServerConfig, which performs the
  	// check for one of every algorithm.
  	srv.HostSigners = append(srv.HostSigners, 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.