Not an issue

Then how is [1,3] a subsequence of [1,2,3] as stated in the example?

Hi, I'm the original author of this Kata (Ruby version). Author of the haskell version didn't understand this Kata properly, and indeed added a DFS version instead of BFS.

I have learned haskell to fix this. I added a BFS solution and a correct buildTree (thanks to @nickie for providing one). Please take a look again.

Thanks,
Karol

It isn't a set, but a sequence/list. The title "power set" is a little bit misleading. You need to return the set of all subsequences, not the set of all subsets, e.g.

--      2nd zero        2nd zero
--       v               v
power [0,0] = [[], [0], [0], [0,0]]
--     ^            ^
--     1st zero    1st zero

This has been reported several times, but the author isn't active anymore. See the issues by @muesli4, @karoyakani, @knotman90, @Thom and @pcapriotti.