Thanks for your answer, dop2000.
Unfortunately, my knowledge of construct is still poor.
I've figured out a solution: to force portions of the alphabet in each value (the first letter is "a", second is "e" or "i", third is "o" or "u", and the same with the consonants, forcing blocks of 3 letters for each value). It's a bit of a drab idea, but it works.
The idea of removing letters from the array seems interesting, but I don't know how to do that. :S