docs: function firstExist(fileNames) {
    const head = fileNames.slice(0, 1);
    if (head.length === 0) {
      return new Promise((resolve, reject) => {
        resolve(false);
      });
    }
  
    const tail = fileNames.slice(1);
    return fileExist(head[0]).then((result) => {
      if (false === result) {
        return firstExist(tail);
      }
  
      return result;
    });
  }