diff --git a/sum.cpp b/sum.cpp
index efd1b27..b0268be 100644
--- a/sum.cpp
+++ b/sum.cpp
@@ -1,4 +1,5 @@
 #include <cassert>
+#include <sstream>
 #include <string>
 
 using namespace std::string_literals;
@@ -7,6 +8,37 @@ 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...);
 }
+
+class ss_sum {
+  std::stringstream ss;
+
+public:
+  template <typename T> ss_sum(T obj) : ss{} { ss << obj; }
+  ss_sum(ss_sum const &rhs) : ss{} {
+    std::string tmp;
+    const_cast<ss_sum &>(rhs).ss >> tmp;
+    const_cast<ss_sum &>(rhs).ss << tmp;
+    ss << tmp;
+  }
+  friend ss_sum operator+(ss_sum const &lhs, ss_sum const &rhs) {
+    ss_sum ret = lhs;
+    ss_sum tmp = rhs;
+    std::string a;
+    tmp.ss >> a;
+    ret.ss << a;
+    return ret;
+  }
+  friend bool operator==(ss_sum const &lhs, ss_sum const &rhs) {
+    ss_sum inner_l = lhs;
+    ss_sum inner_r = rhs;
+    std::string l;
+    std::string r;
+    inner_l.ss >> l;
+    inner_r.ss >> r;
+    return l == r;
+  }
+};
+
 int main(void) {
   bool dummy = sum<double, int, int, int>(1.5, 2, 3, 10) == 16.5;
   assert(dummy);
@@ -17,4 +49,9 @@ int main(void) {
   dummy = sum<std::string>("asdf"s, "abcdefghijklmnop"s, "qrstuvwxyz"s) ==
           "asdfabcdefghijklmnopqrstuvwxyz"s;
   assert(dummy);
+
+  // ss_sum is only associative when everything is ss_sum
+  auto tmp = sum<ss_sum, ss_sum, ss_sum>(1, 2, 3);
+  dummy = tmp == ss_sum{"123"};
+  assert(dummy);
 }