refactor classes, add private, minor fixes

1-js/09-classes/07-instanceof/1-strange-instanceof/task.md

@@ -0,0 +1,20 @@
+importance: 5
+
+---
+
+# Strange instanceof
+
+Why `instanceof` below returns `true`? We can easily see that `a` is not created by `B()`.
+
+```js run
+function A() {}
+function B() {}
+
+A.prototype = B.prototype = {};
+
+let a = new A();
+
+*!*
+alert( a instanceof B ); // true
+*/!*
+```