diff --git a/diff.js b/diff.js
index 8c42dad..67015d0 100644
--- a/diff.js
+++ b/diff.js
@@ -54,6 +54,7 @@ var DIFF_TYPES = new Set([
 // 	zip(func, array, array, ...)
 // 		-> [func(i, [item, item, ...]), ...]
 //
+// XXX revise...
 var zip = function(func, ...arrays){
 	var i = arrays[0] instanceof Array ? 0 : arrays.shift()
 	if(func instanceof Array){
@@ -79,8 +80,9 @@ var zip = function(func, ...arrays){
 		// done...
 		: [] }
 
+
 // XXX should we handle properties???
-// XXX use zip(..)...
+// XXX use zip(..)???
 var _diff_items = function(diff, res, A, B, options, filter){
 	// JSON mode -> ignore attr order...
 	var kA = Object.keys(A)
@@ -152,6 +154,8 @@ var _diff_item_order = function(diff, res, A, B, options, filter){
 
 
 // get common chuncs (LCS)...
+// XXX handle sparse arrays correctly...
+// 		...now empty slots get filled with undefined...
 var getCommonSections = function(A, B, cmp, min_chunk){
 	cmp = cmp || function(a, b){
 		return a === b || a == b }
@@ -230,6 +234,9 @@ var getCommonSections = function(A, B, cmp, min_chunk){
 // 		],
 // 		...
 // 	]
+//
+// XXX handle sparse arrays correctly...
+// 		...now empty slots get filled with undefined...
 var getDiffSections = function(A, B, cmp, min_chunk){
 	// find the common sections...
 	var common_sections = getCommonSections(A, B, cmp, min_chunk)
@@ -352,6 +359,8 @@ function(A, B, options, cache){
 			|| (diff(a, b) == null) }
 
 	// Array...
+	// XXX handle sparse arrays correctly...
+	// 		...now empty slots get filled with undefined...
 	// XXX check seen -- avoid recursion...
 	if(A instanceof Array && B instanceof Array){
 		var res = {