For a it doesn't matter, I just didn't think too much when coding the formula. But for r 0 should be excluded and 1 included.

I'm actually more surprised that xenia managed to come up with the exact same solution I have, even naming of variables, and the error you have pointed out.

Your code should report error on invalid code. invoke(, b) is invalid, invoke(a, ) as well.

Are you asking for spoiling hints?

You didn't even pass the first few tests. Make sure that you read the bnf syntax carefully.

I've just checked the description. The tests are correct.