473. Sequences of Dominoes. A boy had a complete set of dominoes up to double 9, and he was struggling to arrange the tiles in a single line in the usual way - 6 to 6, 3 to 3, blank to blank, and so on. But his father told him:
- You are trying to do the impossible; however, if you allow me to remove four tiles, you will be able to achieve your goal, and the tiles I take will contain the smallest number of points possible under the circumstances.

Which tiles did the father choose? Remember, a regular set of dominoes ends with double 6, but we are considering a set extended up to double 9.
473. If we remove four tiles \(7-6, 5-4\), \(3-2, 1-0\) from the set, the remaining tiles can form a correct sequence. Any other combinations of these numbers would also work; for example, we could remove \(7-0, 6-1, 5-2\) and 4-3. The general rule is that from a domino set ending with a double of an odd number, we must remove tiles that collectively contain all numbers from the blank to the number two units less than the highest number in our set.