fixed compile error due to explicit op and added some tests for bitwise xor which tests things well enough I think

integer.cpp

@@ -272,7 +272,7 @@ public:
     Integer discard = rhs;
     std::stringstream to_output{};
     while (discard != Integer{0}) {
-      int a = discard % Integer{10};
+      int a = static_cast<unsigned long>(discard % Integer{10});
       to_output << a;
       discard /= 10;
     }
@@ -313,6 +313,10 @@ int main(void) {
   assert(a / b == Integer{-2});
   a = std::numeric_limits<long long>::max();
   b = a;
+  for (unsigned long i = 0; i < 20; i++) {
+    Integer n = i;
+    assert(((unsigned long)n ^ i) == 0);
+  }
   for (std::size_t i = 2; i < 20; i++) {
     a += b;
     assert(b * Integer{static_cast<long long>(i)} == a);