code: function (variables, bundleHandler) {
  		// handle if bundleHandler specified in first argument
  		if (!bundleHandler && typeof (variables) === 'function') {
  			bundleHandler = variables;
  			variables = null;
  		}
  
  		return through2.obj(function (file, enc, cb) {
  			if (!checkFile(file, cb))
  				return;
  
  			var ext = path.extname(file.path).toLowerCase();
  			var resultFileName = path.basename(file.path, ext);
  
  			var bundleFiles = processBundleFile(file, ext, variables, bundleHandler, cb);
  			if (file.sourceMap)
  				bundleFiles = bundleFiles.pipe(sourcemaps.init());
  			bundleFiles
  				.pipe(concat(resultFileName))
  				.pipe(pushTo(this))
  				.on('end', cb);
  		});
  	}
docs: concatenates files from bundle and replaces bundle file in current pipe first parameter is function that handles source stream for each bundle