check mine... it has some regex in it :D

Thanks for your suggestion. I added a similar test to all 4 languages. :)

Test for filterS method should also check for monoticty.

You mean that it preserves the original elements that satisfy the predicate?

Lost a lot of time here. Filter can return the same stream all the time (as long as the head passes the condition) and all the tests are green but primes takes ages and never completes. Was really hard to find this bug without debugger.

Sorry to hear that. I will add a test. Nevertheless, you might want to write tests on your own (I now, most people hate it). ;)