public function removeHashPrefixesFromList($list)
  {
    $this->debugLog('removeHashPrefixesFromList() list '.$list);
    $listdir=$this->getListStorageDir($list);
    if(!$listdir || strlen($listdir)<10){
      $this->warningLog('removeHashPrefixesFromList() invalid list directory for list '.$list);
      return;
    }
    if(!is_dir($listdir)){
      $this->debugLog('removeHashPrefixesFromList() no list directory for list '.$list);
      return;
    }
    if(!is_file($listdir.$this->prefixes_filename)){
      $this->debugLog('removeHashPrefixesFromList() no prefix file for list '.$list);
      return;
    }
    unlink($listdir.$this->prefixes_filename);
  }