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In a trump contract, the play of a higher card followed by a lower one may be made from a doubleton , indicating a desire to ruff, rather than just from a holding including a high card.
a doubleton ace
Additionally, all singleton, doubleton , and tripleton sites (variants that appear, respectively, only once, twice, or three times in the total sample) were verified by reamplification and resequencing.
A quatraton is inferred either from the presence of two singletons, no doubletons, and no tripletons or from the presence of no singletons, one doubleton , and no tripletons.
The distribution of singletons, doubletons (low frequency variants), and other (high frequency) variants among the continents was investigated.