docs: function _extractRealFiles (dir, givenFiles, realFiles, callback) {
  
  			if (0 >= givenFiles.length) {
  				return callback(null, realFiles);
  			}
  			else {
  
  				const file = join(dir, givenFiles.shift()).trim();
  
  				return isFileProm(file).then((exists) => {
  
  					if (exists) {
  						realFiles.push(file);
  					}
  
  					_extractRealFiles(dir, givenFiles, realFiles, callback);
  
  				}).catch(callback);
  
  			}
  
  		}