revision #1: 更新第二章中已维护的代码

code/2/todo/fold.expression.cpp

@@ -0,0 +1,8 @@
+#include <iostream>
+template<typename ... T>
+auto sum(T ... t) {
+    return (t + ...);
+}
+int main() {
+    std::cout << sum(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) << std::endl;
+}