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When people use the brush, the position that bear the force is the front
part of it, which will soon dismantle that part, leaving the rest part unused.
The "double" is an advanced version according to the fact above. When the
front part was used up, you can move the front part backward to make the
brush reused, so that you can put an end to waste.
Sui Tingmeng, Xie yasong, Ye Wenjin, Chen Chao