anchor.diff:  
     ###### implementation
   
  -  config = mkIf config.services.postgresql.enable {
  +  config = mkIf cfg.enable {
   
       assertions = [
  -      { assertion = cfg.groupAccess -> builtins.compareVersions cfg.package.version "11.0" >= 0;
  +      { assertion = cfg.groupAccess -> versionAtLeast cfg.package.version "11.0";
           message = ''
             'groupAccess' is not available for PostgreSQL < 11.
           '';
  
positive.code:         default = "";
          description = ''
            Defines the mapping from system users to database users.
          '';
        };
  
  
negative.code:           ''
              # Initialise the database.
              if ! test -e ${cfg.dataDir}/PG_VERSION; then
                initdb -U ${cfg.superUser} ${lib.concatStringsSep " " cfg.initdbFlags}
                # See postStart!
                touch "${cfg.dataDir}/.first_startup"
              fi
  