我唔直接去答你，用例子答。上面講左乜野，自己參透下啦。

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B

A

C

K

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BOOSTBACK BOOSTBACK BOOSTBACK v

BOOSTBACK BOOSTBACK BOOSTBACK v

BOOSTBACK BOOSTBACK BOOSTBACK c

BOOSTBACK BOOSTBACK BOOSTBACK c

BOOSTBACK BOOSTBACK BOOSTBACK x

BOOSTBACK BOOSTBACK BOOSTBACK x

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BOOSTBACK BOOSTBACK BOOSTBACK ;

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resspoon

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