I agree with you. Took me 10 mins to figure out what the task was.. But we get real life problems unclear as this :)

Exactly

I didn't realize that people may not think about that. I've tried to clearify that in the description now. If there's something else that can easily be missunderstood, just tell me and I'll add it too.

Plus one.
And what does it mean "except for Haskell"?

I made the exact same mistake. The description does not mention order is important. I was left thinking that maybe X should map to X if it existed in b, but could map to something else if not. That wasn't it either. Then I read the comments, and discovered the actual intent of the kata.

All this might be in the definition of "isomorphic". But I don't know that. I think the kata would be better if the conditions were explained better.

Ah, thanks. (Sorry, I marked it as an issue before revising it after thinking I'd probably just missed something. Forgot to untag it)

That's not an issue, that's a question.

Also, order is important.

The description of the problem could be made clearer. I'm familiar with complementary DNA and I was still unsure. Here's a suggestion:

In DNA strings, the symbols "A" and "T" are complements of each other, as are "C" and "G". You are given a sequence and your function should return the complementary sequence. The sequence is given as a string (except for Haskell). For example, given the sequence string "ATTGC" your function should return the complementary sequence string "TAACG". The sequence you are given will never be empty.

Of course real-world requirements are often unclear so maybe it's just fine as it is :)