We compress two neighboring vectors in the first two layers and three neighboring vectors in the remaining layers.
for quatrains with seven characters, we use C 1,2 , C 2,2 , C 3,3 , C 4,3 to merge vectors in each layer (see Figure 3); and for quatrains with five characters we use C 1,2 , C 2,2 , C 3,3 .