anchor.diff:            };
   
             prune.prefix = mkOption {
  -            type = types.str;
  +            type = types.nullOr (types.str);
               description = ''
                 Only consider archive names starting with this prefix for pruning.
                 By default, only archives created by this job are considered.
  -              Use <literal>""</literal> to consider all archives.
  +              Use <literal>""</literal> or <literal>null</literal> to consider all archives.
               '';
               default = config.archiveBaseName;
               defaultText = literalExpression "archiveBaseName";
  
positive.code:               determined by <option>dateFormat</option>, will be appended. The full
                name can be modified at runtime (<literal>$archiveName</literal>).
                Placeholders like <literal>{hostname}</literal> must not be used.
              '';
            };
  
  
negative.code:     }
      trap 'on_exit' INT TERM QUIT EXIT
  
      archiveName="${if cfg.archiveBaseName == null then "" else cfg.archiveBaseName + "-"}$(date ${cfg.dateFormat})"
      archiveSuffix="${optionalString cfg.appendFailedSuffix ".failed"}"
      ${cfg.preHook}
    '' + optionalString cfg.doInit ''
  