1. use every digit except 9 (e.g. 0, 1, 2, 3, 4, 5, 6, 7, 8) 2. no using the number 9 SEQUENCE: 0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 20 and so on.
