some properties of scope

doc/design/scope.md

@@ -8,15 +8,29 @@
     1. Scope是Variable的Container
     2. Scope可以共享
 
-## Scope 是一个Variable的Container
+## Scope is a Container of Variables.
 
-解释下为啥Scope是Variable的container。解释下面几个小点的原因。
+    * Scope contains Variables as it's data member.
+    * Scope contains methods that are used to manage Variables, such as Create/Get/Delete.
+    * every variable only belong to one certain Scope.
+    * Scope should destruct all Variables within it when itself is destructed.
+    * Variable can only be created by Scope.
+    * Variable can only be got from Scope.
+
+    * Scope do not contains Operators and have no information to run them.
+
+```cpp
+class Scope {
+ public:
+  Variable* CreateVariable(const std::string& name);
+  const Variable* GetVariable(const std::string& name) const;
+  bool DeleteVariable(const std::string& name);
+
+ private:
+    std::unordered_map<std::string, std::shared_ptr<Vairable>> variable_map_;
+};
+```
 
-    * 他只包含variable
-    * 每一个variable也只属于一个Scope
-    * 每一个Scope析构的时候，会同时析构variable
-    * 只能通过Scope创建Vairable。
-    * 只能通过Scope获取Variable。
 ## Scope 可以被继承或者叫共享
 
 解释下Scope如何被共享，如何查找Variable的算法。