not thread-safe

[ilka-schulz]

As this library does not guarantee thread-safety unless the called function function is itself thread-safe, please consider adding a disclaimer. I saw your disclaimer

DISCLAIMER - This implementation uses threads not a queue

but I think it does not nearly meet the danger that this library poses, maybe specifically mention data races and link to Wikipedia or so. I can see frustrated beginners already because they get non-deterministic behavior and impossible to debug bugs...