Explain assert

ConflictSet.cpp

@@ -1541,6 +1541,7 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
   void setOldestVersion(int64_t oldestVersion) {
     this->oldestVersion = oldestVersion;
     Node *prev = firstGeq(root, removalKey).n;
+    // There's no way to erase removalKey without introducing a key after it
     assert(prev != nullptr);
     while (keyUpdates-- > 0) {
       Node *n = nextLogical(prev);