code: function done( error ) {
  		if ( error ) {
  			return clbk( error );
  		}
  		count += 1;
  		if ( count === len ) {
  			clbk( null, concat( out ) );
  		}
  	}
docs: end FUNCTION next() 
  FUNCTION: done( error )
  Callback invoked upon completing all requests.
  
  @private
  @param {Error|Null} error - error object
  @returns {Void}