Before merging two nodes, we check to see whether the keyword lists associated with them share words with other nodes.
supposed we just merged nodes A and B, and they are expanded with the same syntactic rule into A 1 A 2 ...A n and B 1 B 2 ...B n respectively; before we merge each A i with B i , we check for each B i if its keyword list shares common words with any A j (j = i).