Lately I've been working towards 100%[1] test coverage in RandN. This has been very helpful and has already found several small bugs across the library. One of the RNGs I'm testing is ThreadLocalRng, which RandN guarantees is thread-safe by use of a ThreadLocal - a wrapper around ChaCha that maintains exactly one instance per thread. Another test already verifies that a unique instance is created for each thread, but there could still be some sort of hidden dependency within ChaCha, so I want to test it even further, even though threading is notoriously difficult to test reliably.