Long awaited augmentation of the :map command. Surrounding code needs cleanup.

--HG--
extra : rebase_source : 6bb2406343f04aff20456e1a9d0cb595f4ec767d

common/modules/base.jsm

@@ -937,6 +937,17 @@ const array = Class("array", Array, {
      */
     compact: function compact(ary) ary.filter(function (item) item != null),
 
+    /**
+     * Returns true if each element of ary1 is equal to the
+     * corresponding element in ary2.
+     *
+     * @param {Array} ary1
+     * @param {Array} ary2
+     * @returns {boolean}
+     */
+    equals: function (ary1, ary2)
+        ary1.length == ary2.length && Array.every(ary1, function (e, i) e == ary2[i]),
+
     /**
      * Flattens an array, such that all elements of the array are
      * joined into a single array: