Remove unused and superceded custom methods from XSet class.

js/SSResultSet.js

@@ -222,7 +222,7 @@ class SSResultSet{
           }
           let ctxIds = ctx.in;
           let ctxSet = new XSet(ctxIds);
-          let intersection = ctxSet.xIntersection(activeContextIds);
+          let intersection = ctxSet.intersection(activeContextIds);
           return (intersection.size > 0);
         });
         // If there are no contexts left, then

js/StaticSearch.js

@@ -1187,7 +1187,7 @@ class StaticSearch{
       if (xSets.length > 0){
         let result = xSets[0];
         for (var i=1; i<xSets.length; i++){
-          result = result.xIntersection(xSets[i]);
+          result = result.intersection(xSets[i]);
         }
         result.filtersActive = true;
         return result;

js/XSet.js

@@ -14,10 +14,8 @@
 /** @class XSet
   * @extends Set
   * @description This class inherits from the Set class but
-  * adds some key operators which are missing from that class
-  * in the current version of ECMAScript. Those methods are
-  * named with a leading x in case a future version of ECMAScript
-  * adds native versions.
+  * adds a single property and a convenience function for 
+  * adding items directly from an array.
   */
   class XSet extends Set{
 /** 
@@ -36,38 +34,7 @@
                              //between filters-active-but-no-matches-found
                              //and no-filters-selected.
   }
-/** @function XSet~xUnion
-  * @param {!XSet} xSet2 another instance of the XSet class.
-  * @description this computes the union of the two sets (all
-  * items appearing in either set) and returns the result as
-  * another XSet instance.
-  * @return {!XSet} a new instance of XSet including all items
-  * from both sets.
-  */
-  xUnion(xSet2){
-    return new XSet([...this, ...xSet2]);
-  }
-/** @function XSet~xIntersection
-  * @param {XSet} xSet2 another instance of the XSet class.
-  * @description this computes the intersection of the two sets
-  * (items appearing in both sets) and returns the result as
-  * another XSet instance.
-  * @return {XSet} a new instance of XSet only the items
-  * appearing in both sets.
-  */
-  xIntersection(xSet2){
-    return new XSet([...this].filter(x => xSet2.has(x)));
-  }
-/** @function XSet~xDifference
-  * @param {XSet} xSet2 another instance of the XSet class.
-  * @description this computes the set of items which appear
-  * in this set but not in the parameter set.
-  * @return {XSet} a new instance of XSet only the items
-  * which appear in this set but not in xSet2.
-  */
-  xDifference(xSet2){
-    return new XSet([...this].filter(x => !xSet2.has(x)));
-  }
+
 /** @function XSet~addArray
   * @param {!Array} arr an array of values that are to be added.
   * @description this is a convenience function for adding a set of