properly drop mutually-referring declarations that are not otherwise

referenced and have no side effects

lib/utils.js

@@ -190,3 +190,15 @@ function mergeSort(array, cmp) {
     };
     return _ms(array);
 };
+
+function set_difference(a, b) {
+    return a.filter(function(el){
+        return b.indexOf(el) < 0;
+    });
+};
+
+function set_intersection(a, b) {
+    return a.filter(function(el){
+        return b.indexOf(el) >= 0;
+    });
+};