diff --git a/sum.cpp b/sum.cpp
index b0268be..851f6db 100644
--- a/sum.cpp
+++ b/sum.cpp
@@ -20,6 +20,16 @@ public:
     const_cast<ss_sum &>(rhs).ss << tmp;
     ss << tmp;
   }
+  ss_sum &operator=(ss_sum const &rhs) {
+    std::string tmp;
+    // clear out string stream
+    ss >> tmp;
+
+    const_cast<ss_sum &>(rhs).ss >> tmp;
+    const_cast<ss_sum &>(rhs).ss << tmp;
+    ss << tmp;
+    return *this;
+  }
   friend ss_sum operator+(ss_sum const &lhs, ss_sum const &rhs) {
     ss_sum ret = lhs;
     ss_sum tmp = rhs;
@@ -54,4 +64,10 @@ int main(void) {
   auto tmp = sum<ss_sum, ss_sum, ss_sum>(1, 2, 3);
   dummy = tmp == ss_sum{"123"};
   assert(dummy);
+
+  // demo of situation where associativity isn't applicable ss_sum+(int+int) !=
+  // (ss_sum+int)+int
+  tmp = sum<ss_sum, int, int>(1, 2, 3);
+  dummy = tmp == ss_sum{"15"};
+  assert(dummy);
 }