point about inherit:
1：subclass can get SuperClass`s message
2：subclass rewrite SuperClass`s message
3：many subclass Share a piece of memory
the inherit plan in javascript
1：subClass get superClass by __proto__
2：subClass can reWrite superClass
3：when subClass initialize，is must initialize superClass by call
the principle about instence
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current instence keeping find the attrbute by __proto__,such as:
var a = new A(); a instence Object?
1：a.__proto__ != Object.prototype
2：a.__proto__ -> A.prototype so we judge：A.prototype.__proto__ == Object.protype
so a.__proto__.__proto__ == Object.protype, so a instence A is true