anchor.diff:    ###### implementation
   
     config = mkIf (cfg != null) {
  +
  +    assertions = singleton {
  +      assertion = (cfg == "med_power_with_dipm") -> versionAtLeast kernel.version "4.15";
  +      message = "med_power_with_dipm is not supported for kernels older than 4.15";
  +    };
  +
       services.udev.extraRules = ''
         SUBSYSTEM=="scsi_host", ACTION=="add", KERNEL=="host*", ATTR{link_power_management_policy}="${cfg}"
       '';
  
positive.code: 
      powerManagement.scsiLinkPolicy = mkOption {
        default = null;
        type = types.nullOr (types.enum [ "min_power" "max_performance" "medium_power" ]);
        description = ''
          SCSI link power management policy. The kernel default is
          "max_performance".
        '';
      };
  
  
negative.code: 
  with lib;
  
  let
  
    cfg = config.powerManagement.scsiLinkPolicy;
  
    kernel = config.boot.kernelPackages.kernel;
  
    allowedValues = [
      "min_power"
      "max_performance"
      "medium_power"
      "med_power_with_dipm"
    ];
  
  in
  
  {
    ###### interface
  