impl for sum (maybe) but still need tests for it

sum.cpp

@@ -0,0 +1,7 @@
+#include <iostream>
+
+template <typename T> auto sum(T final) { return final; }
+template <typename T, typename... Ts> auto sum(T first, Ts... rem) {
+  return first + sum<Ts...>(rem...);
+}
+int main(void) { std::cout << sum<double, int, int, int>(1.5, 2, 3, 10); }