After a sunday of hard work, I've evolved the Combos level and I've made a MasterMind clone.
The rules are so simple that fit in a envelope, and you'll find them in the first level. If you still aren't sure about the mechanics, take a look to the tapes.
There are five levels: the first two use only 4 keys, the others use 8 keys.
Levels 1 and 3 use enemies' graphics for the keys, the others use digits.
The last level is slightly easier than the third and the fourth, as seeing the CEs lets you guess the positions of the keys.
The result of each try will show up into five or six seconds, you should never lose a result check (I've spent hours just to let it work every time without waiting too much seconds), but if you wait longer and the result doesn't show up, well, you'll have to restart the level (and by the way it would mean that my efforts were not enough

Enjoy!

http://www.zomis.net/rnd/download.php?id=423