code: func validateOSes(c *validation.Context, cfg *configPB.OSes) {
  	// Operating system names must be unique.
  	// Keep records of ones we've already seen.
  	names := stringset.New(len(cfg.OperatingSystem))
  	for _, os := range cfg.OperatingSystem {
  		switch {
  		case os.Name == "":
  			c.Errorf("operating system names are required and must be non-empty")
  		case !names.Add(os.Name):
  			c.Errorf("duplicate operating system %q", os.Name)
  		}
  	}
  }
docs: // validateOSes validates oses.cfg.