Update interface

doc/design/scope.md

@@ -76,9 +76,12 @@ private:
 using VariablePtr = std::weak_ptr<Variable>;
 
 class Scope {
-public:
+private:
   Scope(const std::shared_ptr<Scope>& parent = nullptr);
 
+public:
+  static std::shared_ptr<Scope> Create(const std::shared_ptr<Scope>& parent = nullptr);
+
   // return nullptr if not found.
   VariablePtr GetVariable(const std::string& name) const;
 
@@ -102,6 +105,8 @@ The `VariablePtr` is a `weak_ptr`. `Net` and `Op` can only get a Variable from `
 
 Local scope contains a `parent_` pointer. It is a linked-list for scopes. Using a `shared_ptr` because when a local scope is using, its parents cannot be destroyed.
 
+Also, as the parent scope is a `shared_ptr`, we can only `Create()` a scope shared pointer. We cannot construct a scope variable, because it cannot be passed to other scope as `parent` pointer.
+
 ## Orthogonal interface
 
 `GetVariable` will return `nullptr` when `name` is not found. It can be used as `Contains` method. `CreateVariable` will return a `Error` when there is a name conflict locally. Combine `GetVariable` and `CreateVariable`, we can implement `CreateOrGetVariable` easily.