Update code

doc/design/scope.md

@@ -5,21 +5,49 @@
 Scope is a context to manage Variables. It mainly contains a map from Variable name to Variable. Net will get and update variable throw scope.
 
 ```cpp
-class Scope {
-    Variable GetVar();
+class Variable;
+using VariablePtr = std::shared_ptr<Variable>;
+
+class Scope final {
+public:
+  Scope();
+  Scope(const std::shared_ptr<Scope>& parent);
+
+  //! Get Variable in this scope.
+  //! @return nullptr if no such variable.
+  const VariablePtr& getVar(const std::string& name) const;
+
+  //! Create or get a variable in this scope.
+  VariablePtr& createOrGetVar(const std::string& name);
 
 private:
-    // var_name -> var
-    std::map<string, Variable> var_map_;
-    Scope* parent_scope_;
-}
+  /// variable name -> variable
+  std::unordered_map<std::string, VariablePtr> vars_;
+  std::shared_ptr<Scope> parent_{nullptr};
+};
 ```
 
 You need to specify a scope to run a Net. One net can run in different scopes and update different variable in the scope. If you did not specify one, It will run in a default scope.
-```python
-with ScopeGuard(scope)：
-    Net net = Net();
-    Net.run()
+
+```cpp
+Scope global;
+auto x = newVar("X");  // x is created in scope global, implicitly.
+auto y = newVar("Y");
+Net net1;
+net1.addOp("add", {x, y}, {x});  // x = x + y;
+net1.run();
+
+for (size_t i=0; i<10; ++i) {
+  Scope local;
+  auto tmp = newVar("tmp");  // tmp is created in scope local.
+  Net net2;
+  net2.addOp("add", {x, y}, {tmp});
+  net2.run();  // tmp = x + y;
+}
+
+Net net3;
+net3.addOp("add", {x, y}, {"tmp"});  // error! cannot found "tmp" in global scope.
+
 ```
 
 ### Chain structure