GPQ Guessing Formula

(item C is correct item IC is incorrect)
Guess 1:
Four of item X

Guess 2:
Four of item Y

Guess 3:
Four of item Z

(you now know what number or items are in the combo)

Guess 4:
Four of 1 item   Three of 1 item   Two of 1 item   One of each item
      |                |                |                |
   C C C C         X X X Y         C C IC IC        (Good Luck)
                  /                     |    \
Guess 5:          /                      |     \
                /                       |      \
               /                   1 correct   2 correct
              /                         |           |
      X X Y X                      IC C C IC    X X Y Z
          |                         /      \           \
Guess 6:   |                        /        \           \
          |                1 correct       2 correct    continued
          |                    |               |            |
      X Y X X              C IC C IC         Y X X Z      X X Z Y
          |                /      \               |    
Guess 7:   |               /        \              |
          |         0 correct   2 correct         |                
          |             |            |            |
      Y X X X       Y X Z X      X Y X Z       Z X X Y
                   (pray that you picked
                     the right spot for
       the other two items)