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