If I understand the problem correctly, then it's much like a problem that plagues most hashmap/associative array implementations: when an object that is used as

If I understand the problem correctly, then it's much like a problem that plagues most hashmap/associative array implementations: when an object that is used as a key is modified (without removing and re-adding it around the modification), you'll often not be able to find it again. It takes some debugging to find this is happening, but in five years of writing Java and Python, I've only had it happen once in either language.