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Basil has several domino tiles, as shown in the figure. He wants to arrange them in a line according to the well-known "domino rules": in any two tiles that are next to each other, the squares that touch must have the same number of points. What is the largest number of tiles he can arrange in this way?  [insert picccc] 
Overseas Competition->Knowledge Point->Combinatorics->Logical Reasoning
One possible configuration for 5 dominoes is shown.  [pic] 