Add Scope Parent & Local section

doc/design/scope.md

@@ -17,11 +17,34 @@
     * 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。
-       * Scope永远从本地寻找Variable，找不到会从他的父亲Scope寻找Variable
-    * 嵌套深度不做要求。
+## Parent scope and local scope
+
+Just like [scope](https://en.wikipedia.org/wiki/Scope_(computer_science)) in programming languages, `Scope` in the neural network also can be local.  There are two attributes about local scope.
+
+*  We can create local variables in a local scope, and when that local scope are destroyed, all local variables should also be destroyed.
+*  Variables in a parent scope can be retrieved from that parent scope's local scope, i.e., when user get a variable from a scope, it will search this variable in current scope firstly. If there is no such variable in local scope, `scope` will keep searching from its parent, until the variable is found or there is no parent.
+
+```cpp
+class Scope {
+public:
+  Scope(const std::shared_ptr<Scope>& scope): parent_(scope) {}
+
+  Variable* Get(const std::string& name) const {
+    Variable* var = GetVarLocally(name);
+    if (var != nullptr) {
+      return var;
+    } else if (parent_ != nullptr) {
+      return parent_->Get(name);
+    } else {
+      return nullptr;
+    }
+  }
+
+private:
+  std::shared_ptr<Scope> parent_ {nullptr};
+};
+```
 
 # 接口实现