function createCallback( idx ) {
		var file = list[ idx ];
		var k = idx + 1;
		return onWalk;
		/**
		* Callback invoked upon walking a file.
		*
		* @private
		* @param {(Error|null)} error - error object
		* @returns {void}
		*/
		function onWalk( error ) {
			if ( errFLG ) {
				return;
			}
			if ( error ) {
				debug( 'Encountered an error when walking file: %s (%d of %d). Error: %s', file, k, len, error.message );
				errFLG = true;
				return cb( error );
			}
			debug( 'Successfully walked file: %s (%d of %d).', file, k, len );

			count += 1;
			debug( 'Walked %d of %d files.', count, len );
			if ( count === len ) {
				return cb( null, toArray( cache ) );
			}
		}
	}
Returns a callback to be invoked upon walking a file.

@private
@param {NonNegativeInteger} idx - index
@returns {Callback} callback