simplify

lib/include/aocpp/Counter.hpp

@@ -18,17 +18,17 @@ struct Counter
   Counter() : n{0} {}
   Counter(std::size_t n) : n{n} {}
 
+  // prefix increment
   auto operator++() -> Counter&
   {
     n++;
     return *this;
   }
 
+  // postfix increment
   auto operator++(int) -> Counter
   {
-    auto temp = *this;
-    n++;
-    return temp;
+    return n++;
   }
 
   auto operator*() const -> EmptyRef