code: func PromoteStableServers(autopilotConfig *Config, health OperatorHealthReply, servers []raft.Server) []raft.Server {
  	// Find any non-voters eligible for promotion.
  	now := time.Now()
  	var promotions []raft.Server
  	for _, server := range servers {
  		if !IsPotentialVoter(server.Suffrage) {
  			health := health.ServerHealth(string(server.ID))
  			if health.IsStable(now, autopilotConfig) {
  				promotions = append(promotions, server)
  			}
  		}
  	}
  
  	return promotions
  }
docs: // PromoteStableServers is a basic autopilot promotion policy that promotes any
  // server which has been healthy and stable for the duration specified in the
  // given Autopilot config.