[reidrac]

I think ['a', 'a', 'ab'] is also valid, isn't it? So ['aa', 'ab'] would be one of the correct answers.

(of course ['a', 'a', 'a'] it's not because the 'b' is not a valid word and it's not in the solutions)

[rfurmani]

Ah right, I was a bit too quick to challenge. Dictionary: aa, aaa, aaaa, ab Word: aaaab