refactor classes, add private, minor fixes

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

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+Yeah, looks strange indeed.
+
+But `instanceof` does not care about the function, but rather about its `prototype`, that it matches against the prototype chain.
+
+And here `a.__proto__ == B.prototype`, so `instanceof` returns `true`.
+
+So, by the logic of `instanceof`, the `prototype` actually defines the type, not the constructor function.

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

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+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
+*/!*
+```