docs: function writeTile(file, url, request, callback) {
    // Result of request
    let res = null;
  
    // FileWriteStream
    let fileStream = fs.createWriteStream(file);
  
    // Register finish callback of FileWriteStream
    fileStream.on('finish', () => {
      callback(null, res);
    });
  
    // Register error callback of FileWriteStream
    fileStream.on('error', (err) => {
      callback(err, res);
    });
  
    // Request object
    let req = request.get(url);
  
    // Register error callback of request
    req.on('error', (err) => {
      callback(err, res);
    });
  
    // Register response callback of request
    req.on('response', (response) => {
      res = response;
    });
  
    // Start download
    req.pipe(fileStream);
  }